Read file info from .hdr file when using -Bs in azgcorr

[mggr]

When run with external bil/bsq files, azgcorr currently requires a -B parameter to specify lines, bands, data type and scaling factors. It'd be nice if the lines, bands and type could come from the .hdr file, with scaling factors being defaulted or set by some other option.

Apparently not currently done as the external file handling is a quick addon for AKW at the minute. This will be worth fixing in the next batch of small tweaks, as it'll make handling atmospherically corrected data a lot easier.