Updated adjoint model impulse forcing and climatology mixing

[arango]

Added a new C-preprocessing option AD_IMPULSE to control the forcing of the adjoint model as an intermittent impulse (on) or a continuous force term (off). Many thanks to Andy Moore for coding this option.

I also added the missing code for the CLIMA_TS_MIX option in the tangent linear, representer, and adjoint models. Recall that this option is used for the horizontal diffusion of tracer perturbation (t-tclm).