split makeplots into separate plotting functions within same file

Fixes #4 for now. Rather than splitting out into separate exampleN.py
scripts,scripts, keep in same file to reuse code setting up output paths and
importing galsim with sys.path hack.

Mostly wholesale copy-pasting from main into new functions, this
could stand to be cleaned up more and modelConstraintPlot fails,
but tbut now these plots are more isolated from one another.