Take default Python version from NEST installation

[kappeld]

I would suggest to change the default behavior of the installation script to take the Python version from the nest installation. nest-config recently introduced the feature to access the python version and path, so it should be quite easy to adapt the cmake script. Since in all practical cases the python version of SPORE and NEST should be the same this is more convenient for most users. We can still allow to overwrite the default using `-Dwith-python'.

see:

nest-config  --python-executable # print full path to Python interpreter used
nest-config  --python-version     # print Python version string for interpreter