This implements a dune-functions grid function using
an old style dune-fufem basis.

Update CI config to clang-7

See merge request agnumpde/dune-fufem!45

The old image is no longer updatet

Fix compilation with Python 3

See merge request agnumpde/dune-fufem!44

At least on my installation, the method PyUnicode_AsUFT8
return const char* rather than char*.

Feature deprecate reference element helper

See merge request agnumpde/dune-fufem!43

Since the functionality is now provided by the reference elements
directly, this is no longer needed.

This matches how to specify search directories for optional
dependencies in DUNE's core modules.

This addresses the following compiler warnings from GCC:

```
../dune/fufem/test/refinedsimplexgeometrytest.cc:74:37: warning:
catching polymorphic type ‘class Dune::NotImplemented’ by value [-Wcatch-value=]

../dune/fufem/test/refinedsimplexgeometrytest.cc:103:37: warning:
catching polymorphic type ‘class Dune::NotImplemented’ by value [-Wcatch-value=]

../dune/fufem/test/refinedsimplexgeometrytest.cc:136:37: warning:
catching polymorphic type ‘class Dune::NotImplemented’ by value [-Wcatch-value=]

../dune/fufem/test/refinedsimplexgeometrytest.cc:151:37: warning:
catching polymorphic type ‘class Dune::NotImplemented’ by value [-Wcatch-value=]

../dune/fufem/test/dunepythontest.cc:183:32: warning:
catching polymorphic type ‘class Dune::Exception’ by value [-Wcatch-value=]
```

Also implement the boundarydofs method for dune-functions bases

See merge request agnumpde/dune-fufem!42

With this patch, the method works both for old-style dune-fufem bases
and for dune-functions bases directly.  The distinction is done
using some minor SFINAE trickery.

Assembler for global basis transfer matrices

See merge request agnumpde/dune-fufem!40

The method assembles the transfer matrx between fine and coarse grids.
Assumptions are
 - the grids are related but not necessarily direct descendants
 - the global dune-functions basis are scalar, i.e., leaf nodes in the basis tree

For evaluating local functions on other geometries a small wrapper is written.
For chaining sequencial father geometries another small wrapper is written.

Two unit tests:
- first, test wether the transfermatrix interpolates coefficients from corase to fine
  for lagrange basis
- second, test wether a jump over two grid levels can be reproduced by two simple transfers

Disable the serialisation test when clang 6 with c++ 17 is used

See merge request agnumpde/dune-fufem!41

With c++ 17 std::auto_ptr got removed. The boost version 1.62 installed on
our CI server is still using it and disabling it with BOOST_NO_AUTO_PTR
leads to undefined reference errors during linking.

Bugfix: Resize local matrix in DuneFunctionsOperatorAssembler

See merge request agnumpde/dune-fufem!39

Most local assemblers use the size of the local matrix as bounds in
the writing for-loops. If the sizes of the local views may change between elements, using
maxSize() as matrix size will lead to bugs.

Handing over a reference does not work anymore since the gridfactory
returns a unique_ptr

Revert "Derive field type with global helper."

See merge request agnumpde/dune-fufem!38

This reverts commit 506d2de0