Add OSC support

[djperrefort]

A module sndata.osc should be added to provide data access for the Open Supernova Catalog. I envision seperate data access classes for spectroscopic (OSCSpec) and photometric (OSC) data. This naming convention follows that of the Sako et al. 2018 data release classes.

Some potential challenges:

1. Querying the OSC can be done through the official REST API. However, I'm unclear on how to specifically query data for only spectroscopically or photometrically observed targets.
2. Formatting the data tables may prove difficult if the data model is not already uniform. However, it may be possible to draw from works that have already analyzed the OSC for machine learning classification.
3. Registering filters may not be a realistic goal for the OSC given how many transmission functions are represented in the OSC.