SCUBA2: add SUBSCAN parameters

New parameters for assessing subscan noise: SUBSCAN_METHOD and SUBSCAN_ESTIMATOR

Also use a little coderef for populating the namespace for related
parameters rather than writing out the same map command each time.
SCUBA2: establish consistent defaults, and delete subscan maps

The method and estimator parameters can be set independently but the
defaults should be consistent. Method tends to override estimator.

Delete subscan maps immediately so it can be done quietly without the
user seeing a list of 200 files they don't need to know about.