Improvements to UI of the catalog manager
Added method to load catalogs into memory from explicit filename input
Convenience attributes added to Catalog_Manager object
Minor modifications to API of load_catalog method of Catalog_Manager
processed_snapshot object now correctly talks to catalog_manager
Integrated optional download behavior into processed_snapshot object
Modified default download behavior of processed_catalog object
Added method to attach simulation characteristics to processed_snapshot object
HOD_mock object now gets simulation data via processed_snapshot
Modified test_Zheng07_mock to use new catalog_manager behavior