Forcing normalized to get a constant energy injection rate

Created originally on Bitbucket by paugier (Pierre Augier)

Was already merged in Bitbucket before import, marked as merged by the import user

The goal is to be able to normalize a forcing scheme forcing in one variable such that the injection rate of another quadratic quantity is constant, for example for ns2d to force in vorticity and to normalize the forcing such that the injection rate of energy is constant.

It allows one to compare different forcing schemes keeping the same injection rate of energy.