Update MEGAN extension

1. Update initialization phase to not define certain species (ISOP, ACET,
   ALD2, C2H4, LIMO, MTPA, MTPO, PRPE, SESQ, SOAP, SOAS, EOH) when the
   OFFLINE_BIOGENICVOC option is turned on in HEMCO_Config.rc. This will
   allow use of offline biogenic VOC emissions for those species and online
   MEGAN emissions will be used for the rest of the species. Right now, just
   MOH from MEGAN will be fed back into the model and the remaining
   monoterpene species, methyl butenol, formic acid, and acetic acid will
   be used for diagnostic purposes only.

2. Remove support for MEGAN_Mono extension and just use MEGAN extension.
   Monoterpenes are now fully implemented in all full-chemistry simulations
   and emissions for those species should always be turned on when using
   MEGAN. This also avoids confusion having two extensions within a single
   extension module.

3. Add manual diagnostics for several monoterpene species, methyl butenol,
   formic acid, acetic acid so that they can be saved out via HEMCO_Diagn.rc.
   These manual diagnostics were formally defined in GeosCore/hcoi_gc_diagn_mod.F90
   for saving out to BPCH but were removed with BPCH removal.

4. Move FLUX* arrays to extension instance

5. Clean up revision history comments and remove obsolete and unused code.

Signed-off-by: Melissa Sulprizio <[email protected]>