add forcing with the following formula:

$f_x = (1 - A) \sin k_0 y + (k_1 / k_0)^3 A \sin k_1 y$

The prefactor is there to (somewhat) ensure that the Reynolds number
doesn't change drastically when modifying A.

not tested yet.

code runs without segfaults. I still need to test the correctness.

very simple functionality, but I gain two lines per value in the useful
file, which I consider a big deal.

This reverts commit b4bbc44b.

This reverts commit 6c803fa1.