hack searching 2 catalogs

[gibsongreen]

Description

Hack Hour PR, follow is to-do:

• Enable multi-select for catalog drop down in UI
• Denote max search per catalog? Or general totally to possibly load into Imviz
• Break up catalog.search() function: _query_sdss, _query_gaia, _query_from_file and so on so they are individually callable in catalog.search()
  Modify catalog.search(): Handle cases of 1 catalog versus multiple catalogs, Add hidden column to denote which source came from which catalog, Ensure columns are either standardized when loaded if columns from different catalogs vary or accept all columns
• investigate cross-matching algorithms

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?