Revert "Merge branch 'master' of https://github.com/yade/trunk"

This reverts commit c48d742b2924916740891fa2572bd956b821f693, reversing
changes made to 0bb8ad658256c7ae2d8abbb5d0a55293b642e997.
Updated bubble behaviours in BubbleMat.hpp and BubbleMat.cpp
	- surfaceTension now part of Law2
	- force-displacement curve transitions to exponential
	  based on a critical value (for stability)
	- computeForce correctly uses the separation value (negative)
	  instead of the positive penetration depth