Merge branch 'cleanup/move-COMPARE_RECORD-into-UGDIM-namespace' into 'master'

ref:staging/dune-uggrid This silences a compiler warning from GCC when
building programs with LTO where GCC sees two definitions of COMPARE_RECORD
(for 2d and for 3d):

    ../gm/refine.cc:3450:8: warning: type ‘struct compare_record’ violates the C++ One Definition Rule [-Wodr]
    ../gm/refine.cc:3450:8: note: a different type is defined in another translation unit
    ../gm/refine.cc:3453:12: note: the first difference of corresponding definitions is field ‘elem’
    ../gm/refine.cc:3453:12: note: a field of same name but different type is defined in another translation unit
    ../ug/gm.h:1644:24: note: type ‘union ELEMENT’ should match type ‘union ELEMENT’

See merge request [!118]

  [!118]: gitlab.dune-project.org/staging/dune-uggrid/merge_requests/118

This silences a compiler warning from GCC when building programs with
LTO where GCC sees two definitions of `COMPARE_RECORD` (for 2d and for
3d):

```
../gm/refine.cc:3450:8: warning: type ‘struct compare_record’ violates the C++ One Definition Rule [-Wodr]
../gm/refine.cc:3450:8: note: a different type is defined in another translation unit
../gm/refine.cc:3453:12: note: the first difference of corresponding definitions is field ‘elem’
../gm/refine.cc:3453:12: note: a field of same name but different type is defined in another translation unit
../ug/gm.h:1644:24: note: type ‘union ELEMENT’ should match type ‘union ELEMENT’
```

Update CI config for master

See merge request !117

Switch to the new CI image location and the new way of selecting toolchains. The builds are mostly
identical to the core modules, and I also kept the additional sequential build, which does not
require shipping a custom opts file anymore.

Add missing includes for compilation with libc++

See merge request !116

libc++ doesn't pull in some of the headers implicitly included by libstdc++, causing compilation to
fail. This adds the required explicit `#include` statements.

Resolve "dune-uggrid won't compile with Clang 6.0 (Ubuntu 18.04)"

Closes #43

See merge request !114

This fixes #43

fix issues when building dune-uggrid as shared library

Closes #42

See merge request !113

Closes: #42

More cleanup

See merge request !112

Most bits were already removed in eb65f980

More cleanup

See merge request !111

The default-constructor for forward_list was only made non-explicit in
C++11 via a defect report. This is however not implemented in all
supported older compilers; for example GCC 5.4 does not implement
this.

The function is no longer needed since 9e4946b0

This addresses the following compiler warning from GCC 8:

```
../parallel/ddd/xfer/pack.cc:104:13: warning:
‘bool UG::D2::sort_MsgSize([...])’ defined but not used [-Wunused-function]
```

This addresses the following compiler warning from GCC 8:

```
../parallel/ddd/xfer/cmds.cc:346:21: warning:
extra tokens at end of #undef directive
```

This addresses the following compiler warnings from GCC 8:

```
../parallel/ddd/xfer/cmds.cc:302:12: warning: format ‘%ld’ expects
argument of type ‘long int’, but argument 4 has type ‘int’ [-Wformat=]

../parallel/ddd/xfer/cmds.cc:327:12: warning: format ‘%ld’ expects
argument of type ‘long int’, but argument 4 has type ‘int’ [-Wformat=]
```

This addresses the following compiler warning from GCC 8:

```
../gm/algebra.cc:2081:9: warning:
unused variable ‘dddContext’ [-Wunused-variable]
```

This addresses the following compiler warnings from GCC 8:

```
../parallel/ddd/mgr/typemgr.cc:161:8: warning:
unused variable ‘buf’ [-Wunused-variable]

../parallel/ddd/mgr/typemgr.cc:235:15: warning:
unused variable ‘errtxt’ [-Wunused-variable]

../parallel/ddd/mgr/typemgr.cc:492:14: warning:
unused variable ‘errtxt’ [-Wunused-variable]

../parallel/ddd/xfer/cmdmsg.cc:390:8: warning:
unused variable ‘buf’ [-Wunused-variable]

../parallel/ddd/xfer/cplmsg.cc:368:8: warning:
unused variable ‘buf’ [-Wunused-variable]

../parallel/ddd/xfer/cmds.cc:1160:16: warning:
unused variable ‘descDepTyp’ [-Wunused-variable]

../low/fileopen.cc:136:13: warning:
‘OldBasePath’ defined but not used [-Wunused-variable]
```

The variables are not used in all cases which caused the following
compiler warnings from GCC 8:

```
parallel/ddd/if/ifcmd.ct:199:16: warning:
unused variable ‘tries’ [-Wunused-variable]

parallel/ddd/if/ifcmd.ct:200:16: warning:
unused variable ‘recv_mesgs’ [-Wunused-variable]

parallel/ddd/if/ifstd.ct:95:16: warning:
unused variable ‘tries’ [-Wunused-variable]

parallel/ddd/if/ifstd.ct:96:16: warning:
unused variable ‘recv_mesgs’ [-Wunused-variable]
```

[cmake] Require CMake 3.1

See merge request !90

DDD: move global state to a context object

Closes #36

See merge request !96

This makes it easier for a patch to dune-grid to support context
objects optionally.

Otherwise the tags assigned to elements and those to DDD will not
match: the (global) tags assigned to elements will increment for each
new MG. After a new MG is created, the mapping to already existing
tags used by DDD will break as the MG-side will use new tags.

The local rank is only known from the `PPIFContext` which is not
available here.