.cmd files and tpagb_norm_type parameter

This pull request adds the tpagb_norm_type setup parameter, and will therefore break for FSPS revisions prior to r153. I updated the accepted revisions list to reflect this.

I also included a method in the StellarPopulation object for writing  .cmd files (detailed isochrone info) and then reading that data into largish arrays. This is a bit hacky but should work with any future updates to the FSPS write_isochrone subroutine.