replace opaque error message with something easier for user

[StuartLittlefair]

Fixes #159 by adding a helper routine to crop calibration frames.

Note the nice error message needs to know the type of calibration frame being applied, which leads to this slightly verbose syntax. It would be good if there was a nice way of removing this duplication without adding code to the MCCD class, but I don't think there's a way to infer the calibration type automatically.

[martinjohndyer]

I see what you mean about the syntax, something like calib_type = calib_frame.head["DTYPE"] would catch most cases if you're using bias/flat/dark etc files from the cameras, but I'm guessing that wouldn't work for fmaps etc since makefringe doesn't seem to set it, or any header cards like TARGET for that matter. Maybe the make* scripts probably should write something to the output header to mark it as a calibration file (other than just a comment), but that's a separate issue.