Add doctest for MKSHomogenizationModel
Add doctest for MKSHomogenizationModel

Address issue#143

Add doctest for MKSHomogenizationModel. The supporting text needs to be
developed along with the doc tests for each of the methods.

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