Add pressure support to Bridge

[rieder]

Is your feature request related to a problem? Please describe.
Bridge currently deals directly with forces, i.e. depending on a particle's position only (with #556 adding support for velocity-dependent forces).
It might be good to also add support for pressure to Bridge, i.e. a force that also depends on the particle's surface (or radius if that is easier).

Describe the solution you'd like
A transparent modification to Bridge in a similar manner to #556, where we add the surface (or radius) of particles as an optional attribute for calculating kicks.

[rieder]

I can solve this in another, much easier way.