Added material model based on rheology in Morency and Doin, 2004
Merge pull request #158 from jperryhouts/morency_doin_material

Added material model based on rheology in Morency and Doin, 2004
Fix minor typo in a comment.
Merge pull request #238 from jperryhouts/fix_minor_typo

Fix minor typo in a comment.