introduced Python definition

if python is found in CMake, this definition selectively
activates/deactivates the Python-dependent parts.
revisited python/udunits cmake options/c++ definitions

using for udunits a cmake option

splitted python related options into two:
  - python backend (only uses python library, for e.g. python expressions)
  - python library (usses additionally the swig library for building python library)
fixed python library

reimplemented Swig python interface file
updated python tutorial: all test cases are covered introduced in 'functionbackend-demo.cpp'