Precompute time-dependent forcing term

[peterrum]

PR #1270 made the assumption that the forcing function is constant over time. However, it seems that it can be time-dependent

lethe/source/solvers/fluid_dynamics_matrix_free.cc

Lines 2019 to 2020 in 64527c5

	this->forcing_function->set_time(
	this->simulation_control->get_current_time());

We need a logic to that allows us to figure if a function is time-dependent or not.

[blaisb]

I'll take care of that.