OCSADV-191: pass an obs context to the magnitude table lookup instead of creating the magnitude table with an obs context
OCSADV-191: thread the MagnitudeTable through all the code that ultimately needs it
OCSADV-191: add parsing of probe limits from a file, loading magnitude table at startup
OCSADV-191: switch to the config file based mag table
OCSADV-191: separated the GeMS-specific behavior in DefaultMagnitudeTable to GemMagnitudeTable, deleted DefaultMagnitudeTable, delegated to GemsMagnitudeTable where necessary.
OCSADV-191: minor cleanup
OCSADV-191: handle empty entries in the spreadsheet
OCSADV-191: switch to probe limits downloaded from spreadsheet
OCSADV-191: log exceptions
Merge pull request #90 from swalker2m/feature/OCSADV-191

OCSADV-191: configuration by magnitude table.  Merging for continuing work on AGS.