Update some logic in SU2G_GridCompMod

[pcolarco]

These are several enhancements captured here:

• Sulfate surface area density calculation in SU_Compute_Diags was incorrectly being passed the effective radius used for settling along with the sigma width of the number distribution. Properly it should be passed the number median radius, also present in the RC file. Added a hook to read that field from the RC file ("particle_radius_number"), store in SU grid comp, and pass to SU_Compute_Diags. This change is zero-diff to the SU internal state. It changes value of export SO4AREA.
• Add some protective logic around reading daily volcanic emission files. If filename does not exist model will write message to standard output and reset volcanic emissions to zero. This is zero difference result unless in a place where the files were not present.
• Changed DMS concentration data holder from ExtData provided (SU_DMSO) to local copy (dmso_conc). This is relevant since if we run source tagged instances where we don't want DMS emissions we would zero out dmso_conc and that is what should be passed to DMSemission subroutine. This is zero diff except in that case.

[pcolarco]

changes committed to branch: feature/pcolarco/#263/SU2G_GridCompMod_logic