wrap import galsim in try/except statement

reduce dependency on galsim by handling failure to import with a
boolean flag hasGalSim. Checking hasGalSim could be done more gracefully, perhaps by changing convOpt from galsim to pixel with a warning statement.

Also removed global pars imgSizePix and pixScale in sim.py, replacing
them with Model attributes. Need to update this in plot.py
bug fix in else syntax