Automatically query the latest GSC

[shanosborne]

We should set up this code to automatically query the latest GSC, without having to update the default value of the catalog parameter in utils.query_gsc().

According to Brian McLean, there isn't a way to programmatically find out the newest catalog version right now (either to get that number exactly, or get a full list of all catalogs that can then be sorted), but it is an issue he wants to address. So as soon as that functionality is available, it should be implemented here.

The other option is to not specify a catalog number to just use the default, but right now the default is GSC23 which isn't the most up to date version. So if the default catalog is going to be updated, that would also be a fix for this issue. See the documentation on the default catalog here: http://gsss.stsci.edu/Software/WebServices.htm