Consider Refactor of STACCatalogSearch class

[ptomasula]

Background

The STACCatalogSearch class includes a bunch of custom tooling for iterating over a STAC catalog including a IteratorHandler and multiple calls to get_assets get_children and get-items. While this is working just fine, it seems like the walk method would allow for much of this code to be simplified and would leverage the iterators provided by the library.

See issue #15 for additional discussion.