[FEATURE REQUEST] Replace space-delimiter requirement in HISTORY.rc

[lizziel]

Name and Institution (Required)

Name: Lizzie Lundgren
Institution: Harvard University

New GEOS-Chem feature or discussion

This feature request is to replace the use of spaces in diaglist_mod.F90 to string-split lines when reading HISTORY.rc. Requiring a space in the file can trip up users, as in #2270. With the feature the following line should not cause the model to crash:
SpeciesConc.fields:'SpeciesConcVV_NO2','GCHPchem',

Alternatively, we could require whitespace but allow tabs. However, if MAPL allows no whitespace then we should as well.