Forcing/drag for primitive models too

[milankl]

• Moves JetDrag and StochasticStirring from StochasticStir.jl into the main repo @KristianJS
• StochasticStirring now depends on model.random_process* @KristianJS
• calls to forcing! and drag! in the primitive equation models as part of the dynamics, after the parameterization, before the other dynamics (i.e. always accumulate tendencies, otherwise parameterizations are overwritten.

(*) e.g. random_process = SpectralAR1Process(spectral_grid, wavenumber=N, time_scale=T, ...). Default NoRandomProcess so this needs to be explicitly passed to the model constructor when using StochasticStirring. This way we can reuse the random_process infrastructure for SPPT etc too. And in fact, the defaults of SpectralAR1Process are more geared towards that (randn with std 1/3 clamp at (-1, 1)).