force each run to the same affine in realign4d
making realign truly consistent with runs with different affines
add test
Merge pull request #253 from alexis-roche/fix-multisession-realign

extend realign4d to deal with runs with different affines
RF: update .travis.yml to use pip for sympy
BF: use sphinx inheritcance_diagram extension

We had our own copy, but it was old, and finally new sphinx lost some
API that it needed.  Delete our copy and use the sphinx version.
DOC: fix broken citation in docstring

It was causing a sphinx crash.
ENH: visualization deals with empty maps

Not crashing on empty maps is useful, as they pop up every once in a
while