updated mix_by functions so density is updated
added test_multimaterial2 to verify correct density, modified test1
removed extraneous debug statements
added description of the test and asserted against hand calculated values
fixed merge conflict
pep8 fixes
pep8 naughtiness
pep8 naughtiness
Merge pull request #330 from makeclean/staging

Update of material mix_by functions such that density is updated accordingly.