Commit replace int / long by int32_t / int64_t and adds explicit
instantiations for these two types where they were missing. Also
we convert all real_t to double.

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

Commit adds syncVectorFunctionBetweenPrimitives() which can be used to
synch P[12]VectorFunction objects.

since interpolate only operates on owned dofs we dont need communication

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.

clang is of course right "C++ requires a type specifier for all
declarations" aka void was missing.

This commit fixes the issue with the P1ElementwiseMassOperatorTest test.
As pointed out by Daniel the problem was actually not with the implemen-
tation of the operator itself, but with the fact that the micro-coordinates
had not been synchronised between primitives.

- added a new file Syncing.hpp which implements a function to synchronise
  arbitrary hhg_functions between all primitives called
  syncFunctionBetweenPrimitives()
- added synchronisation in P1ElementwiseMassOperatorTest
- removed dead code from applyElementwise() in VertexDofMacroEdge()