move basic model functionality into sourcemodel classes
import extinction module from specutils
WIP on branch model-refactor
WIP on model-refactor
WIP on model-refactor
WIP on model-refactor
WIP on model-refactor
update version to v0.3.0 for release
Update changes and whatsnew for v0.3
update relevant files for v0.4 development
Merge branch 'model-refactor'

Conflicts:
	sncosmo/snanaio.py