Consolidating ORM class definitions

ephem/build_master_table.py
 - Added assert statements to get_fits_file.
 - Updated the doc string for get_fits_file.
ephem/database_interface.py
 - Pulled the MasterImage and Finders class into here to create a single authoritative representation. Created a brief session to allow the classes to be defined.
ephem/run_ephem.py
 - Moved the MasterImage and Finders class definitions into database_interface.py