First attempt at making a constructor for HelioProjective frames.
This is to prevent it from defaulting to a UnitSpherical rep if zeta is not specified.
It defaults zeta to 0, in a way similar to the Heliographic frames.
Merge branch 'coordinates-framework' of github.com:VaticanCameos/sunpy into coordinates-framework

Conflicts:
	sunpy/coordinates/frames.py