Fix calculation of massR in ViscoelasticPM

Thanks to Dominik Boemer for pointing this out.
http://www.mail-archive.com/yade-users@lists.launchpad.net/msg08741.html
Fix cs calculation in ViscoelasticPM

It seems to be an error in [Pournin2001] (22) Eq.4, missing factor 2
Thanks to Dominik Boemer for pointing this out
http://www.mail-archive.com/yade-users@lists.launchpad.net/msg08741.html