Followup to #216 for GaussianPSF amplitude

This PR makes the `GaussianPSF` `amplitude` parameter really be the peak amplitude of the PSF and not mean the total integrated flux.  It seems that given the functional form of `GaussianPSF` defined with error functions (I'm not familar with this form) it really wants the total integrated flux as the parameter.  In any case, I continued @adonath's PR (#216) to use `amplitude` as the input parameter.