What's the point of neglecting the computation of tendencies when taking a time-step?

[glwagner]

The docstring for time_step! says that "Setting compute_tendencies=false will not calculate new tendencies ". But why would anybody want to do that? Since tendencies are computed at the end of a time-step rather than before, this would prevent one from taking a subsequent time-step. I think this should be clarified in the docstring. (It's also missing a period.)

Oceananigans.jl/src/TimeSteppers/quasi_adams_bashforth_2.jl

Line 70 in 1ee317b

Setting `compute_tendencies=false` will not calculate new tendencies

cc @simone-silvestri