Many changes towards a more consistent sign convention throughout the code for stresses and strains. Should fix e.g. https://bugs.launchpad.net/yade/+bug/1381282. The goal is to know directly the meaning of a strain or stress value each time one is encountered. Relying on the classical Continuum Mechanics convention (editorial choice...). A global announcement is planned in a couple of days, in case some errors appear in the meantime.