Updated registration_chain.py so that stats are now calculated from each subject to the average time point. This is in addition to the existing calculations between pairs.
1. All stats calculations moved to concatAndResample, which was renamed to LongitudinalStatsConcatAndResample.
	a. Changed from function to class to better handle increased complexity.
	b. Additional array added to keep track of xfms to average (in addition to common space).
	c. Added calculation of stats from each time point to average, then resampled to common space. Final time point handled separately and only if not average.
	d. General cleaning up of code and creating of functions to eliminate duplicate functionality.
	e. Looping over blurs now happens in resampleToCommon.

2. Moved transform and inverseXfm out of statsGroup, since they aren't really necessary there.
3. Removed subjectStats dictionary from registration_chain. Again, not really necessary.
4. Updated minc_displacement defaults so that if there is no specified xfm, it gets the last xfm between input and target. Removed targetFH as necessary file in input array.