endpoint scaling: change default threephase_model to gwseg

and throw an exception if "simple" is encountered...

According to Ove, gwseg should be used, because "gwseg is the model
relevant to the norne case - i.e the model eclipse uses.

The fix for the simple model has to wait for a refac of the satfunc
complex."
Merge pull request #529 from andlaus/change_default_threephase_model

endpoint scaling: change default threephase_model to gwseg