Substantial speed up for the first step.
 * we were defining exposures by reading headers from the flt files every time
 * now we define exposures by reading the _epochs.txt file, when available
 This is much faster.
bugfix for refim definition.
 * now correctly finds an existing refimage, regardless of the _drc or _drz suffix
Bugfix for ACS single_sci creation.
 * fix for bug from assuming '_flt.fits' on images with '_flc.fits'
add --singlesci
 * effectively just equivalent to --singlesubs
added cleaning level 5 (the nuclear option)
 * removes every possible intermediate file, but can cause a race condition