Swap problem in InteractionLoop

When you use two different materials, it might happen that IntreactionLoop ends with error [1]. It is because of IPhys stage (see also the comment in [1], but in reality Ip2 can by non-symmetric).

Possible solutions:

1. define both Ip2_Mat1_Mat2 and Ip2_Mat2_Mat1 (according to [2] maybe does not work..)
2. replace assert [1] (something like if (swap) { I->functorCase.phys=physDispatcher->getFunctor2D(b2->material,b1->material,swap); swap=false }). Is the assert necessary at all?
3. make option 1) somehow automatic (using some macro?)

Pros and cons:

1. would mess the source code and documentation a bit, but otherwise I think it is ok
2. would it have some side effects? Is it ok for Law2 stage? etc etc?
3. maybe the best option if the solution is reasonable

cheers Jan

[1] http://bazaar.launchpad.net/~yade-pkg/yade/git-trunk/view/head:/pkg/common/InteractionLoop.cpp#L116 [2] https://answers.launchpad.net/yade/+question/269315

Original bug report by Jan Stránský on 2015-07-18 : https://bugs.launchpad.net/yade/+bug/1475844