PPs_PBs_contactArea

Hi,

In this MR I push a code to calculate the contactArea for the Potential Particles code, where only a constant-stiffness linear law was available so far. Also, I replaced the boolean parameter calJointLength (worked only for 2-D contacts) in both codes with a boolean parameter calContactArea, which works for 2-D and 3-D simulations. With this, the users are given the option to have a constant-linear stiffness law for the PBs as well.

The code to calculate the contactArea for the PPs is simpler than the one used for the PBs, but requires more iterations per contact, since starting from the contact point, it swipes along the shear direction in fixed-sized angular steps (areaStep), tracing points on the surface of the overlap region, which are used to calculate the contactArea as the area of a closed 2-D slice of the overlap region.

I updated the examples, check-scripts and docs to comply with these changes in syntax.

All the best, Vasileios