New Feature for xform_brain: on-the-fly shortcuts for multi-transform chains

[schlegelp]

Is your feature request related to a problem? Please describe.
Moving data from a source to a target template through multiple transforms can be rather slow.

Describe the solution you'd like
A new option for xform_brain (e.g. shortcut=True) that first generates a landmark-based transform directly from source to target space and then applies that shortcut to all the data.