Add bitmap testing to ipolates/ipolatev tests

[AlexanderRichert-NOAA]

Currently in interp_mod_grib{1,2}.F90, the input bitmap is always zero/false. Let's add at least a test for each grid and interp scheme that will enable some kind of input bitmap in order to test that functionality. The instances I can find in the code of ibi being used are checking whether it's equal to zero, and if it is, the interpolation is applied at that grid point. This means that the tests for non-zero bitmaps should be set up to look for differences that occur when the bitmap is applied, so we know that it is getting used.