Updates to stats calculations:
***This code still needs testing. ***
1. CalcStats now has option argument of an array of file handlers, which are used to set up average_inv_pure_displacement calculation.
2. Calculation of average and subsequent recentering of displacement now included.
3. linAndNlinDisplacement function written for base class.
4. Updated calcFullDisplacement in both CalcStats and CalcChain stats to reflect updates to mincDisplacement atom.
5. Updated mincDisplacement atom for clarity. Now requires source/starting space, transform, and file handler for location of output.
6. Addition of various functions to avoid duplicate code and assist in items 1 and 2 above.

Minor related changes:
1. We can now do a stats calculation from the NLIN module.
2. Creation of a new mincAverageDisp atom, for averaging displacements, which inherits from mincAverage.
3. Added calc_stats (default=TRUE) option in addGenRegOptionGroup

First check-in of several as I continue to test. Addresses Issue #55.