The FunctionWrapper class no longer typedefs FunctionType to its
func_t argument. IMHO that was pretty useless. Instead we get a
```
template< typename VType >
using WrappedFuncKind =
typename WrappedFuncType::template FunctionType< VType >;
```
For this to work functions the get wrapped need to provide a templated
FunctionType. So this is added to
- DGFunction
- and all already exisiting children of the BlockFunction class.

Commit
- makes EdgeDoFFunction::enumerate() public
- adds inclusion of FunctionProperties.hpp in BlockFunction.hpp
- performs a hotfix in DGFunction class, as there was an inconsistency
  between uint_t and ValueType (low level implementation in DGVertex,
  DGEdge and DGFace expects to receive uint_t, but was passed ValueType,
  which fails e.g. for double)

The microsoft compiler was so kind as to try and instantiate a
FunctionWrapper< DGFunction< real_t > >. This failed and demonstrated
that there are further methods missing from DGFunction class that
the other functions sport.

Commit adds those to the list of dummy implementations and extends
the FunctionWrapperTest to try and wrap various kinds of functions
that we currently have in HyteG.

* Makes Function and CSFVectorFunction base classes again
* Marks their children as final
* Removes some superfluous const qualifiers in method interfaces
  and enhances consistency between functions

In order to cleanup 'src/hyteg' a little bit and in preparation of working
on the class hierarchy for functions (see als issue #142) the commit
introduces a new 'functions' subdirectory and move the following files
into it:

 - Function.hpp
 - FunctionIterator.hpp
 - FunctionProperties.cpp
 - FunctionProperties.hpp
 - FunctionTraits.hpp

This adds a multElementwise() method to the DGFunction class and extends
the associated FunctionMultElementwiseTest. For this to work smoothly the
commit also performs the following two updates on the DGFunction:

- it adds an assign() version for a constant value
- it changes the interface of the other assign() method for functions
  to use a vector of reference wrappers instead of a vector of pointers
  so that it is consistent with assign in the other function classes,
  like P1Function etc.

TODO: Implement tests to verify correctness

the enumeration function now only enumerates and is void
to get local and global DoF count use the
numberOfLocalDoFs / numberOfGlobalDoFs functions

Removed access method for communicators in the Function base class.
Replaced them via methods for communication in the derived classes.

This is a first step towards the implementation of alternative
communication routines.

This commit implements DGFunction::getMaxMagnitude() as by issue #70.
Since the DGFunction currently is only of order 0, i.e. we have only
dofs inside microfaces, we only need DGFace::getMaxMagnitude().

Commit also adds one sub-test for the DG case to FindMaxMinMagTest.