Implement load_cfa1 function

[kbarbary]

Based on this code which parses these files.

However, we'd like to download the ascii data files from the original published source rather than including them in the repository. The original data seems to be available from https://www.cfa.harvard.edu/supernova/SNarchive.html under the link "CfA SN Ia light curves from Riess 1999" which comes as a tar.gz archive.

We may eventually want to combine all CfA data into a single load_cfa() function, but this should be a good start.