drizzle.py : drizzle parameters now adjust the pixscale for first-pass drizzle
  according to the number of flt files in the fltlist.

register.py :
   - check for existing wcsname before running tweakreg
   - pass clobber parameter downward (not yet doing anything)

main.py : accommodating the above
sort.py :
   - epoch 00 is now defined as a special template epoch. Observations only
    go into this epoch if they occurred before mjdmin or after mjdmax.  So the
    idea is that mjdmin and mjdmax bracket all the SN epochs, and everything
    else (before and after the SN) goes into epoch 0.
   - after sorting exposures into epochs, they are recorded to a text file
     that defaults to <outroot>_epochs.txt.
     Existing epoch sorting is read from the text file if it exists.

main.py :
   - added a new command line interface with argparse
   - added a new parameter 'doall' to run all the processing steps.
   - bug fixes

test.py :
   - new test module, with a function to run the colfax data through from
     top to bottom.
     (initial test run with F160W produces clean subtractions)