Field-Obstacle interactions?

at the moment (as far as I remember) Field*Containers only support interactions between fields and particles. it seems useful to also allow obstacles to interact with fields, e.g. to set boundary conditions

In theory this is could already be done insided the step! function as fields are by definition everywhere (so there is no performance gain from boxing etc.), but it seems unInPartSian to let one side of an interacting pair do all the work.