No embedded python in pip install lalsuite

Hi @duncanmmacleod,

When installing lalsuite through pip install I cannot use the new waveforms interface to call my own external python model.

Looking at the code it seems that the variable HAVE_PYTHON3_EMBED here is not defined, because I get the error message "Python generator not implemented" in line 18.

Probably when the python wheels are created for pypi one needs to switch on an option to compile with embedded python?

Could you have a look? Thanks!