Repartition to a fixed pixel order

Enable a user to repartition a custom catalog to a fixed pixel order, selected by a user. Under this partioning scheme, the sky is partioned in pixels that have the same sky coverage. This will be useful for crossmatching catalogs (especially sparse one, that cover relatively uniformly large parts of the sky).

Start by locating the custom-catalog repartitioning entry point and any related tests, then determine how a user-selected fixed pixel order should represent equal sky coverage. Done means a custom catalog can be repartitioned using the selected order and supports crossmatching for sparse, broadly distributed catalogs.

Written by the indexing model from the issue text.