NF: add python 3 compatibility routines

For future conversion to python 2 / 3 common codebase.
NF+TST: add slicetiming utility functions

Functions for returning slice times given number of slices and TR.

Keep dictionary of allowable names for lookup of function name and
aliases as string.
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.
RF: switch tests to use SpaceTimeRealign

Assuming SpaceTimeRealign should be preferred to FmriRealign4d
RF: rewrite example using SpaceTimeRealign
DOC: fixes to timefunc docstrings
RF: delete unused test helper function
fix import error