exposures.py bug fixes; and default final imsize is now 30 arcsec

exposures.py bug fixes;
  * when some off-target flts were present, the epoch numbering got screwed up
  * when determining if the source is within the field of view,
  we now allow for a buffer of 50 pixels on ACS and 20 pix on WFC3 (this
  helps to ensure that when a source falls in the ACS chip gap on one
  exposure, we do not erroneously toss out the whole visit)

  in progress :
  * add an initialization function for Exposure objects using a row from the
   _epochs.txt file