Bad Pixel Mask and LAcosmicx

- maskimages() definition/function added. It creates a bad pixel mask
for each flatscience, flatarc, and flatstandard.
- LAxsciences() definition/function added. It runs lacosmicx on each
flat-fielded science image.
- Options added to indivtasks() to run maskimages() and LAxsciences()
individually (instead of only during a full reduction of the data).
- dictionaries() now adds a 'GAIN' keyword to the 0-header of each
image. Based on the PySALT wiki documentation, if 'GAINSET'=='FAINT' and
'ROSPEED'='SLOW', then the gain should be 2.146 so this is what the
pipeline sets 'GAIN' to (if the keyword combination matches up).
- LAxsciences() and apall() and imcombine() are now fed the gain
(default value of 2.146) through this new 'GAIN' keyword.
- Note that the 'GAIN' keyword should be transferred automatically by
PySALT's SALTMOSAIC task but there is a bug which prevents -- so the RU
pipeline does it for you.