Get list of unique products

[snbianco]

Added implementation, a test, and documentation for a new function (Observations.get_unique_product_list). Given observations, the function returns a list of associated data products, each with a unique data URI. This is essentially a wrapper to Observations.get_product_list that removes duplicate rows before returning.

Because get_product_list is a query method, it should be async and return a response that is then parsed into a Table. Because we need the actual result Table to remove the duplicates, I wasn't able to add this functionality as a parameter to the original function. It's possible that the Mast.Caom.Products service could filter out duplicates at some point, but I think this is the best solution for now.

[snbianco]

@dr-rodriguez @astrojimig

[astrojimig]

This looks good! Thanks @snbianco !