SCUBA2: add KEEP parameter to allow recipe-specific suffices
SCUBA2: use tags to denote which data to use to make a map

Use data that is not tagged with "olddata" to make a map. Once that
data has been used, it is tagged with "olddata". This means that the
multi-level use of the "isgood" method can be phased out.
SCUBA2: pick the last sequence start/end values

The code designed to catch short sequences and so trigger map-making
needs to be told to pick the last element of the array to determine if
the sequence is a short one.