Add keyword arguments to MKSHomogenziationModel
Add keyword arguments to MKSHomogenziationModel

Address issue #148

Add keyword arguments periodic_axes and probablity_mask as well as
supporting documentation to the MKSHomogenizationModel. Now this model
can be used to create homogenization relationship with periodic and
non-periodic microstructures and take into account the confidence level
of the data used.

davidbrough1/pymks,fredhohman/pymks,davidbrough1/pymks,XinyiGong/pymks,awhite40/pymks