remove disk rescaling in shearEllipse, add defineEllipse

shearEllipse was changing disk_r by a factor of (1+gamma), which isn't
a proper treatment of magnification. As far as I can tell, this only
impacted imaging visualization because disk_r wasn't used in ellModel
for fitting (and it was hidden from the visualization tests by a fudge
that made fig1b and fig1c in makeplots.py identical)

defineEllipse is a small helper function to produce an ellipse array
from model attributes.
name->modelName, add galName attr to Model class

also rename setDefaultPars to setupAttr to separate "pars" used for
likelihood calculation (and in updatePars) from full list of model
attributes.

galName is introduced to enable making a few standard sets of galaxy
attributes beyond the defaults.
rewrite shearVMapPlot to use Model object rather than par list

Bringing makeplots examples up to date for #14. Added helper function
getShapes to take a model and get (unsheared) ellipse and lines for
plotting visualization.