Added insert_dateobs() to run_astrodrizzle(), to add the DATE-OBS keyword back into the headers of the AstroDrizzle products. For Ticket #154.
Only add DATE-OBS back in for ACS and WFC3. Switched to using convenience function to add keyword back. For Ticket #154.
DATE-OBS reinsertion is now done for WFPC2 as well. For Ticket #154.
Merge pull request #157 from STScI-Citizen-Science/date-obs

Added inserting the DATE-OBS back into into all AstroDrizzle outputs.