Implement fast spatial filter unions and intersections

Feature request

We need an interface to enable multiple spatial filter unions and intersections. This feature provides a lot of flexibility when it comes to sky region selection and it allows for the decomposition of complex polygons to handle, for example, non-convex polygon regions. The following interface suggestion is merely illustrative:

catalog.search((BoxSearch() + ConeSearch()) * PolygonSearch()...)

This also tackles the issue documented in #180.

Before submitting
Please check the following:

• I have described the purpose of the suggested change, specifying what I need the enhancement to accomplish, i.e. what problem it solves.
• I have included any relevant links, screenshots, environment information, and data relevant to implementing the requested feature, as well as pseudocode for how I want to access the new functionality.
• If I have ideas for how the new feature could be implemented, I have provided explanations and/or pseudocode and/or task lists for the steps.

Start by reviewing the existing spatial filter and catalog.search interfaces, then read the related issue #180. Determine the intended semantics for composing BoxSearch, ConeSearch, and PolygonSearch with unions and intersections. Done should include a defined interface that supports the requested complex-region selections.