READ chemistry and radiation output from Athena++. Use example of chem_six_ray regression
test output

Pre-requisit:
- install astropy. e.g. with anaconda, run
  $conda install conda-forge::astropy
- add athena_read.py to PYTHONPATH, e.g., add
    "export PYTHONPATH=$PYTHONPATH:[your path to athena]/athena/vis/python"
    to your .bashrc or .bash_profile file, then restart terminal