ENH: Allow fixed,moving masks in scripts

[cookpa]

This was kind of an undocumented feature because you could do -x fixedMask.nii.gz,movingMask.nii.gz and it would work. The script would change the arg to -x [ fixedMask.nii.gz,movingMask.nii.gz, NULL ].

I changed it so that -x fixedMask.nii.gz,movingMask.nii.gz maps to -x [ fixedMask.nii.gz,movingMask.nii.gz ] and -x fixedMask.nii.gz maps to -x [ fixedMask.nii.gz, NULL ] as before.

[gdevenyi]

May I suggest changing NULL to NOMASK, this may reduce the previously seen bug reports.

[cookpa]

Not easily becasue NULL isn't just a script convention, it has special meaning in the C++ code.

[gdevenyi]

The way the code works in the masking inside antsRegistration seems to be

• check if file exists and is loadable as a mask
• if not loadable use no mask

Ergo, you can create a file called NULL and antsRegistration will use it (ignoring the complications of file format detection...)

[cookpa]

Oh yeah, so it does. It gets the nullptr from attempting to read the file.

I guess then it could be changed. But now I'm thinking it would be better to parse NULL without printing the error message - because the code as written will run without a mask if there is a typo in the mask file name.

[gdevenyi]

This would cause me much pain because I've been using NOMASK for a wide swath of scripts I'd need to update.

I stil support making it error out if it can't read the mask provided.