PICARD: add MOSAIC_EACH recipe parameter

The MOSAIC_EACH recipe parameter is a flag to control whether or not
to combine all the given files or to coadd by object. Default is 0
which means mosaic every file to produce a single output file.
PICARD: add ability to coadd by object

Specifying the MOSAIC_EACH recipe parameter will now cause this
primitive to coadd the files by object, as defined by the FITS keyword
OBJECT. An output file with the suffix _mos will be created for each
object. The default behaviour remains to combine all files into a
single output mosaic.