Adding Calspec spectral library

Calspec need to be added to our set of spectral libraries. I am using getCalspec, the libray developped by Jeremy Neveu.

I have added a module calspec.py in bbf.stellarlib. Since fetching the library takes some time, I have added a caching mechanism, so that the library can be stored as parquet files in ~/.cache/bbf and restored quickly if needed. @betoule @maudelejeune you may have a look if you want.