Calculate and report enorm

[tsalo]

Summary

Idea from @ericearl. I don't know if we'll need to add an --enorm-thresh argument, but at least we can calculate the measure as a first step. We can basically just do np.sqrt(np.sum(delta_params ** 2)), using motion parameters in millimeters. I'd rather calculate the mm-equivalent for rotation parameters than just use degrees like AFNI does.

[ericearl]

Here's the slide I heard about "enorm" from: