Correctly handle units in all interfaces

[effigies]

Many of our interfaces that use img.header.get_zooms() assume mm+s units, rather than checking get_xyzt_units(), while the NIfTI standard also allows for meters, microns, milliseconds and microseconds. There will eventually be a dataset where this bites us, so we should try to address this before it becomes a real issue.

In the short term, it may make sense to create a helper function that takes an image and returns zooms in mm+s, and just call that instead of img.header.get_zooms() (not sure if this belongs more in fmriprep or niworkflows).

In the long term, I've proposed a new method on NIfTI headers that always returns mm+s (nipy/nibabel#567). We can use that as a template for a get_norm_zooms(img) function.