SCUBA2: improved 'smooth' ZPD estimator

Use CUPID FINDBACK instead of KAPPA MATHS abs to take
absolute value of interferogram before smoothing to
get the initial ZPD estimate.  This should provide better
baselining to avoid skewing the Gaussian fit.

The position fitted to the smoothed interferogram is
extracted for manipulation and then re-inserted back into
the NDF before running SMURF FIT1D the second time.
The positions are thresholded about their median
and bad values are filled in using KAPPA FILLBAD.
SCUBA2: calculate box sizes from mm scale

_APPROX_FTS_FRAME_ now puts the pixel per mm scale in a
header so that we can use it to calculate the parameters
for applications which take input in pixel units.
SCUBA2: swap inversion for 450um subarrays

The criterion applied in commit 97f88ab5c6 works for the
850um subarrays, but appears to always have the
smoothed interferogram peak falling between fringes
for the 450um subarrays.  This commit therefore
swaps the inversion criterion for 450um subarrays.
SCUBA2: use smooth estimator in REDUCE_FTS_ZPD

Following the recent changes, this seems to work for the
20121218 commissioning data.
SCUBA2: threshold around median in _FILL_ZPD_MAP_

This primitive now calculates the median value and uses this
to calculate the thresholding limits.