Small updates to group unpack and rats provider checks

[christophkeller]

This PR brings in two small updates that I made in support of GEOS-Chem simulations at higher resolutions. These updates were cherry-picked from branch feature/cakelle2/gcc_rats_provider. This branch has a number of additional updates that don't need to go into develop. The two updates are:

1. Add an optional verbose flag to routine Unpack_Chem_Groups (commit a558d09): If verbose is set to .false., it will stop printing all the N/Cl LOOP COUNT statements, which can clog up the logfile.
2. Expand the logic of the RATS provider connectivities (commit 11522b8): This makes sure that RATS connectivities between two components are only established if that chemistry component is indeed selected as the RATS provider.

[mmanyin]

@christophkeller Two small changes: in GEOS_ChemGridComp.F90 line 572 please change the provider to PCHEM.
Note that it doesn't cause a problem in the standard run because gas and aqueous chem are 'off' in ACHEM by default.

Also, please document these changes the CHANGELOG.md file