polychromeIFS() expects input cube to have a header card with misleading name

[evanbray1]

'input_sampling' is drawn from a header card by the name 'PIXSIZE'. Naively one might think this represents pixel size in meters (because the same header card name is used in the Param class, where the units are in meters). However this instance expects it to have units of Lambda/D.

If it's standard practice for pixel size (regardless of units) to always be found under a header card named "PIXSIZE" I will accept it. Otherwise I propose that header['PIXSIZE'] shall always have units of meters, and that pixel size (in units of Lambda/D) be stored under a different card.

[evanbray1]

We recognize that there are higher levels of redundancy in expected header card names than were known at the time of this issue, and we defer this undertaking to a later effort.