Make more robust Spex Prism loader

[dr-rodriguez]

Currently, the Spex Prism data loader is very basic. It checks that spex is somewhere in the file and then assumes it's a Spex Prism file and proceeds from there. It should be a little more robust, perhaps checking the header contents instead.

Furthermore, the load_spex function needs to better handle cases of missing header keywords, invalid units, and such.

Finally, unit tests should be written for the spex-related functions in spectra.py