mask input cube based on a flag cube provided by the user

With this merge request users can provide a flag cube. The flag cube is a FITS with integer data type, and needs to have the same size as the input cube. The flag cube is then used to flag (i.e., blank) the input cube before any other SoFiA step (e.g., noise measurement, S+C, etc).

This option is part of the flag section of the pipeline. It is more flexible than, e.g., flag.region because the 3D shape of the flagged region can be arbitrarily complex.

A possible application (and the original motivation to implement this) is to mask bright absorption lines detected in a previous run of SoFiA. That prevents those bright absorption lines from "polluting" the reliability calculation or being merged with nearby emission sources.

Once the merged request is approved we should add the new parameter flag.cube to the wiki, and edit the user manual.