instance where boosts were not weighted by course credits 

There are a couple thing to check in the pull request - the first is a simple logical gate for instances where grade_scale is none; this should only affect Balt, but might affect other schools down the road who have prescale=True but no grade_scale set up yet...


Second, and most importantly, there was a situation in StudentYearGrade where the boost was not weighted by course credits and instead by enrollment count; so that's been fixed.

I just need reassurance that this won't affect other schools *other than Baltimore*