Describe provenance of data

[keflavich]

We spent a lot of effort hunting down the awkward origin of these data.

@astrofrog your notes and feedback would be useful here.

@richardson-t I'd like to post your investigations notebook, with updates, as validation of the statements included here.

[astrofrog]

For the record the files in this repo for the atmospheres have been renamed but maybe we could diff the values with those in files