Van der Waals gas density added.  Fixes #3244 .  Doco and test included
Merge pull request #3330 from WilkAndy/methane_3244

Van der Waals gas density added.  Fixes #3244 .  Doco and test included
Merge commit 'ae8dc9d59bf5f0248f02cf0692267d2c41b8ff3f'