Redid the integration functions in galsim/integ.py  There are now two versions each of midpoint and trapezoidal rule; one version for (possibly unevenly spaced) tabulated samples, and one version that does the sampling at `N` equally spaced intervals.  I also scrapped the GK integrator, since it didn't seem to add much.  We can always resurrect it later if desired.

Note that I didn't add Simpson's, since this doesn't trivially generalize to non-uniform spaced samples, though given that scipy.integrate.simps can use non-uniform samples and this [blog post](http://mobiuz.wordpress.com/2011/01/23/numerical-integration-over-non-uniform-grids/) exists, this could probably be added as well.