RF: refuse to load / convert MINC for now

Nibabel can load MINC, but now we need an API to get the axis names for
MINC, otherwise, the axes can be anywhere, but are most likely to be in
the opposite order to the ones the NIfTI needs.  Raise an error until we
have worked out how to do this right.
TST: skip test for non-analyze types for now

In fact it turns out we can't sensibly load the non-analyze types, so
skip this test.
WIP: trying to test the tsdiffana command