Create syntax for custom resolutions

[tsalo]

What would you like to see added in this software?

This relates to #996 and nipreps/fmriprep#1604.

My first-thought proposal:

• Use p instead of . For example, MNI152NLin6Asym:1p5mm is 1.5 mm isotropic.
• Use x to separate dimensions. For example, MNI152NLin6Asym:res-6x6x3mm is 6 x 6 x 3 mm.
• Put mm at the end to distinguish from template's resolution labels. For example, MNI152NLin6Asym:res-03:res-3mm gives you outputs in the res-03 resolution (0.5 x 0.5 x 0.5 mm) and in the custom 3 x 3 x 3 mm resolution.
• Assume isotropic definition if only a single value is provided. For example, res-2mm is the same as res-2x2x2mm.

Do you have any interest in helping implement the feature?

Yes

Additional information / screenshots

No response