Quite important changes in JCFpm code

- formal changes : all mechanical parameters for joint interactions, previously defined as parameters of Ip2..., are now stored in JCFpmMat. Sounds more logical for me and allows setting different joint parameters to different bodies.

- not formal change : the local stifnesses for joint interactions are now computed according to some surface relative to the in-contact spheres (instead of their radii), moreover the corresponding mechanical parameter. This parameter has the unit of Pa/m, rather than Pa, which may sound more logical for interface media.