SCUBA2: background removal is no longer necessary

Maps are much flatter than they used to be and so estimating fcfs from
pointing observations is much more reliable. Similarly for focus
observations, there is no need to remove a background as the maps are
flat.
PICARD: add REMOVE_BACKGROUND recipe parameter

The REMOVE_BACKGROUND recipe parameter controls whether or not
background removal is carried out on a given image. Default is 0: do
not carry out background removal.

A new argument called USEPAR has been added to
_SCUBA2_REMOVE_BACKGROUND_ which dictates whether or not the recipe
parameter should be used to determine background removal. Default is 0
- ignore the recipe parameter.
SCUBA2: add REMOVE_BACKGROUND recipe parameter to the pipeline

Background removal is now turned off by default when calcualting FCFs,
but add the ability to turn it on with a recipe parameter.