Apply the Kolmogorov forcing and drag in the OpenFOAM ns-grid solver #276

The ns-grid OpenFOAM tesseract runs icoFoam, which has no momentum source term. It therefore cannot take the body force and linear drag that #267 adds to the ns-grid schema, and it is excluded from forward/kolmogorov_laminar and forward/kolmogorov (#272) as NOT_IMPLEMENTED.

OpenFOAM can apply both through the foamRun incompressibleFluid solver with an fvModel: a vector source for the force, and an implicit coefficient for the drag. Wiring this up would:

• put OpenFOAM into both Kolmogorov forward checks;
• allow a forward-only entry in the solver-in-the-loop benchmark (feat(domain): add a Kolmogorov-flow solver-in-the-loop benchmark to ns-grid #274): the uncorrected and supervised arms need no solver gradient.

I have not checked which fvModel types the OpenFOAM version in our image supports, or whether switching from icoFoam to foamRun changes the existing ns-grid results. Both need checking first.