Moved some cmake stuff to the cook-off

See issue #5.
Moves PythonPackage and PythonVirtualEnv.
These two packages are used in BEM++ as well (bempp/bempp#71).
Find a coherent python install from cookoff recipe

Goes towards issue #5. The CoherentPython package makes sure (tries to) that the python executable
and the python libraries are the same. This is also used in bempp/bempp#71.