[IMP-dev] More minor patches

Since we are doing the patch game, here are some trivial ones: Add addition and multiplication by a scalar to vector3D (python support needs to be added separately).

Just throw an exception on a bad cast (no error message) in the decorator. Currently catching an exception is the only way to check if a particle has the required fields or not. We may want to add a cleaner mechanism.

Make sure that bonds connect two different particles.