SCUBA2: start adding support for JSA tiling

This commit introduces a new recipe REDUCE_SCAN_JSA_PUBLIC and a
new recipe parameter JSA_TILES.  _MAKEMAP_FRAME_ can be instructed to
produce JSA tiles either via this recipe parameter, or by an argument
-- as is now done by REDUCE_SCAN_JSA_PUBLIC.  It also now uses the
OUTFILES parameter of MAKEMAP to get a list of the files produced.

The reason for adding both the recipe parameter and the argument is
that having the dedicated argument allows the new REDUCE_SCAN_JSA_PUBLIC
recipe to produce JSA tiles automatically (for the JSA "public" products)
while the recipe parameter allows tiles to be requested for other recipes
(e.g. for JSA "project" products).  The recipe parameter introduces
a new uhdr entry rather than adding another MAKEMAP_-prefixed recipe
parameter because this uhdr is used to control more than just the call to
MAKEMAP itself.  Note however that the other recipes will need further
modification to avoid them having problems with JSA tiles -- e.g. they
could skip all processing after tagging the observation product.