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.