Reworking image interpolation to work on mask and variance image
simultaneously with the proper image. Working in the idea of having
"good", "bad" and "poor" output mask pixels (taken from Andy Becker),
where "poor" pixels have at least one bad input pixel contributing to
the flux, but the contribution is small (where 'small' is defined by a
user option). To handle all the options, created a
psImageInterpolateOptions structure, which contains all the things
which are constant while looping over an image (including the input
image, variance and mask). Changed function name, and split code out
of psImage.[ch] into its own file. Function returns boolean error
status; output values for image, variance and mask come from pointers
to double. Added interpolation types (GAUSS, LANCZOS2, LANCZOS3,
LANCZOS4). Using a fairly general framework so that additional
interpolation methods might be easily added. Updated tests and other
dependent code. Tests pass (but only test BILINEAR and BICUBE), and
pswarp works (but currently only uses BILINEAR).
