Resolve "lalinference.imrtgr.imrtgrutils.calc_final_mass_spin failing on master"

Description

This should fix #262 (closed) where bbh_average_fits_precessing() fails when called with scalar inputs. All the lower-level fit functions called by this wrapper through _return_quantity() already do proper vectorizing, so the single sanity check line that is changed in this MR should be the only bit that needs a fix.

API Changes and Justification

Backwards Compatible Changes

• This change introduces no API changes
• This change adds new API calls

Backwards Incompatible Changes

• This change modifies an existing API
• This change removes an existing API

Review Status

@nathan-johnson-mcdaniel can review

Closes #262 (closed)