Generalize `image_counts.py` to take in different inputs from CLI

Currently, we hard-code to a specific TOPAZ file, wavelengths, and time

Generalize so we can define the inputs on the command line.

Example:

image_counts.py --input-file /SNS/TOPAZ/IPTS-31189/nexus/TOPAZ_46917.nxs.h5 --wavelength-min 1 --wavelength-max 3.5 --time 60