Rename and deprecate Observations and ObservationsClass

These classes are way to generic, ideally the class name should keep some reference for the module they are coming from.
It currently affects 2 modules, mast and gemini.

Start by locating the Observations and ObservationsClass definitions and all references in the mast and gemini modules. Review how these public classes are imported and used before deciding the module-specific names and deprecation path. Done means both affected modules use names that identify their source module while the intended deprecated names are handled consistently.

Written by the indexing model from the issue text.