Add an option to read initial species from an external file #2794
Description
Motivation or Problem
Sometimes you want to list more initial species in your model than is convenient to fit in an input file. There should be a way to direct RMG's core initialization to a species dictionary, kind of like a seed mechanism but without any parameter values filled in
Additional Context
This could help speed up RMG runs if you have a workflow that involves changing the libraries or families. You might know a bunch of species that should exist in your model, but for some reason you can't start from an exact seed and don't want to have to waste lots of time rebuilding the model from scratch.