when running on multiple input files, interpolation runs over an increasingly smaller number of rows

[PaulHancock]

When running fits_warp with a single model input, but multiple images, the number of rows which are used in the interpolation stage is decreased by 4x for each image after the first.

This command

fits_warp/fits_warp.py

Line 461 in 76b667b

stride = int( stride// 4)

is executed during every instance of the loop which begins at:

fits_warp/fits_warp.py

Line 409 in 76b667b

for fname in fnames: