Fix bug in variance generation and clean up code
Fix bug in variance generation and clean up code

address #242

Fix bub in variance generation. The previous version of the code could
create too large of a fluctuations in the volume fraction of the last
phase due to randomness. This version fixes that by scaling if
necessary. Clean up code by combining variables and improved notation.

davidbrough1/pymks,awhite40/pymks,davidbrough1/pymks