Make temporary file names clearer

by using a prefix and a suffix.
Scale all the positions in one loop
RIT: Determine best filters to use for match

Reference catalogues tend to contain multiple filter results and
this can sometimes have an unforseen effect if the filter is very
different. Go through the filter list in each catalogue and
choose the closest match.

This code should probably be a method in Astro::Fluxes.
Simplify determination of scale factor

...and use abs() in case there are negatives.