Rename spatial objects from *Search to *Region

[hombit]

Feature request

"Region selection" terminology looks more intuitive for astronomers than "search filtering". I propose to change:

• *Search -> *Region or use astropy regions library classes instead
• Catalog.read_hats(search_filter=) -> read_hats(region=)
• Catalog.*_search -> Catalog.*_region
• Catalog.search -> Catalog.select_region

Related to #664

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