Replace slice_order argument with slice_times in FmriRealign4D

As discussed on the mailing list, the `slice_order` argument of FmriRealign4D is confusing as it has a different meaning than in the SPM slice timing routine. It is also restrictive as it does not enable us to represent irregular slice acquisition schemes. The changes introduced by this PR  make `slice_order` (as well as other additional parameters `tr_slices` and `start`)  obsolete keyword arguments and replace them by a `slice_times` argument, which is a keyword argument for now but will become mandatory in future versions.