Enable variable dimension (2d/3d)

[streeve]

Because of the template on the input class, CabanaPD will now be a header-only library

Enable 2d or 3d simulations; primarily changes to the Particles class. Blocked by #57 which will simplify cell spacing here quite a bit. Also could block #33 since that is set up to be a 2d problem

[pabloseleson]

@streeve : It seems here we are generalizing from 3D to a general dimension Dim. Correct? Note that when changing from 3D to 2D, one has to change the model, for example, from a fully 3D model to a 2D plane elasticity model with a change in the corresponding model constants.

[streeve]

Note that when changing from 3D to 2D, one has to change the model, for example, from a fully 3D model to a 2D plane elasticity model with a change in the corresponding model constants.

Agreed, feel free to add here if it's clear how to do so the force models. But since we don't have any explicit 2d examples right now we could merge that separately