Refactored the code to get rid of some duplication b/w getChip
and getPSM.

Still goes "off in the weeds" when accessing the NITFs in the
pan/color/psm.   Need to adjust for scale and geometry.
Determined that it is the sensor modeling that is causing
the error.  Just switch over to TIFFs and it works okay.

Still need to do scale/geometry based filtering.