SCUBA2: replace isgood check with tagexists

Using specific tags allows much finer control of which files to
select. In this case, the files to be processed should be valid maps
with the "newimage" tag.
SCUBA2: proceed if the Frame is good
SCUBA2: do not set the isgood flag for dark/fastflat Frames

Instead, allow tags to specify which files should be processed.
SCUBA2: do not shift WCS if the input files are already shifted

Since calibrators are now shifted automatically after the map has been
made, there is no need to shift them again at the coadding stage.