New 'functions' subdirectory
In order to cleanup 'src/hyteg' a little bit and in preparation of working on the class hierarchy for functions (see als issue #142 (closed)) the commit introduces a new 'functions' subdirectory and move the following files into it:
- Function.hpp
- FunctionIterator.hpp
- FunctionProperties.cpp
- FunctionProperties.hpp
- FunctionTraits.hpp