Python modules should not be EXPORTed as CMake targets

[StefanBruens]

Python modules (ngpy, ngguipy) are loaded (dl'opened) using pythons own module mechanism (import), but not linked when building any applications or other software packages depending on netgen.

When the python modules are included in the EXPORT set, a file dependency is added to the netgen CMake config, making the netgen python modules a hard dependency, even when only using the netgen C/C++ interface.