allow for non-complete emodels

[arnaudon]

If there are some missing emodels for some etypes, this is crashing:

https://github.com/BlueBrain/BluePyMM/blob/master/bluepymm/prepare_combos/create_mm_sqlite.py#L365

Maybe we could add an option from config to allow for non-complete emodels, and do:
morph_fullmtype_etype_map = morph_fullmtype_etype_map.dropna()
before the check.

[wvangeit]

Yes, this was by design.
If there is a use case, we could an option with a list of e-types that should be ignored.