Move all force and torque calculation into separate function

Discussed and proposed by Bruno.
http://www.mail-archive.com/yade-dev@lists.launchpad.net/msg09742.html
Move capillary functions out of ViscEl-classes.

Discussed here:
http://www.mail-archive.com/yade-dev@lists.launchpad.net/msg09786.html

Thanks Bruno and Klaus.