RF: drafting interface for slice timing.

Make two classes, one for realignment with slice time correction, for
for realignment only.

Simplify options and remove default guess about which is the slice
axis.
RF: move conditional code into if blocks

Timing parameters only used if ``time_interp`` is True; reflect this
with conditional setting of parameters.
NF: raise FutureWarning; check warnings

Raise FutureWarning as well as DeprecationWarning for FmriRealign4d;
check these are raised.