[c++] More `SOMAGroup` refinements 1/5

[nguyenv]

Issue and/or context:

This work is originally a part of https://github.com/single-cell-data/TileDB-SOMA/pull/2675

Changes:

• Replace contents of SOMACollection::get with a call to SOMAObject::open
• Rename SOMAGroup::member_to_uri_mapping to SOMAGroup::members (this is a getter that returns a map of the child member's name and its associated SOMAGroupEntry)
• Refactor SOMAGroup::members mapping to store SOMAGroupEntry values. SOMAGroupEntry stores the URI of the child member and the type of SOMAObject it is. This type info gets passed into SOMAObject::open (via SOMACollection::get) to reduce the number of unnecessary tiledb::open calls
• Add an iterator for SOMACollection
• Add missing documentation
• Rethrow TileDBError as TileDBSOMAError where relevant

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.91%. Comparing base (eab6557) to head (3aeb786). Report is 2 commits behind head on main.

Additional details and impacted files

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