turn off GOSWIM via N_constit=0

[gmao-rreichle]

The present PR attempts to turn off GOSWIM by setting the number of constituents to 0.

GOSWIM is a snow impurity scheme that was implemented in GEOS but has not been used.

Previously, N_constit=9 was hardwired and GOSWIM calculations were executed even though GOSWIM was never used. To turn off GOSWIM, the GCM's rc parameter AEROSOL_DEPOSITION was set to 0, which forced the constituent mass and the deposition rates to remain zero, but the many GOSWIM do loops through the 9 constituents were still executed, thus multiplying and adding lots zeros many times.

If needed, original behavior can be recovered by setting N_constit=N_constit_GOSWIM=9

Remaining tasks:

• verify 0-diff
• quantify improvement in execution speed (if any)
• update AEROSOL_DEPOSITION in GEOS_SurfaceGridComp.rc
• additional cleanup if desired (e.g., make N_constit a configurable input)