Fixed virial contribution in ActionVolume.

This is only important if using non-subcell volumes.  If you use subcell the virial
is zero
Merge branch 'master' of git.assembla.com:plumed2
Merge branch 'master' of git.assembla.com:plumed2
Merge branch 'master' of git.assembla.com:plumed2
Small fix to alphabeta

master multicolvar base is slightly different to v2.0 multicolvar base.
You need an extra funciton in multicolvars that are in master.
Merge branch 'master' of git.assembla.com:plumed2
Fixed a typo in the manual