Simplify model run_env instructions

[theamarks]

Problem / Observation

Many places throughout model scripts to change run_env object values to "local", "aws", or "demo" which can make switching among model runs tedious and error prone. e.g. We are currently not renaming datadir to not break code. Need to manually change among datadir <- "model_inputs" vs datadir <- "model_inputs_sau"

Proposed Solution

• Add README instruction for the 3 model runs:
  • README directly or pointing to a Wiki for "local" and "demo"
  • artis-hpc repo README link for "aws"
• Setup automatic dataoutputs write depending on model version run
• Create shell script to define global environmental variables to define model runs instead of changing many individual lines in the code. Also serve as metadata for model version

Alternatives Considered

• encapsulate scripts within functions to have run_env logic
• Can't remember further alternatives - check written notes