SCUBA2: rename _FILL_ZPD_MAP_ to _FILTER_ZPD_MAP_

Removed the actual filling operation from this primitive.

This will allow the two operations to be separated.
SCUBA2: update _MAKE_MOSAIC_FPLANE_ synopsis.

INFILES and OUT were removed in commit 1340a512ee.
SCUBA2: tag itermediate ZPD fit zpd_pos_estimate

If _FIT_INTERFEROGRAM_PEAK_ is run with the smooth estimator
then it puts the fit to the smoothed interferogram into
the frame and tags it.
SCUBA2: add stack and collapse primitives

Together these can be used to average the ZPD maps by subarray.
SCUBA2: add separate _FILL_ZPD_MAP_ primitive

This contains the filling and indexing functions which were
previously in _FILL_ZPD_MAP_ before it was renamed to
_FILTER_ZPD_MAP.

In addition it now writes the mean and median ZPD position
by subarray to a log.zpd file.
SCUBA2: simplify _DISPLAY_ZPD_ primitive

Also added SUFFIX and TITLE parameters.

Updated the disp.dat file to put the previous plots on the right
hand side of the window, and added _zpdmss plots on the left
(the fit to the smoothed interferogram).
SCUBA2: rename log.zpd to log.zeropath

In order to match the index file name.
SCUBA2: update REDUCE_FTS_ZPD recipe

Remove _SET_FTS_AXIS_ option.

Use the separate _FILTER_ZPD_MAP_ and _FILL_ZPD_MAP_ primitives.

Display initial estimate map.

Collapse subscans by taking the mean.
SCUBA2: update ZPD recipe/primitive documentation