PeMS/cmp_volumes.py changes

[chunhochow]

• If you have time, perhaps it would be better to update it to use argparse to read in a config toml file to set the inputs. See transit/apc/transit_apc_volume_and_speed.py for an example
• Also feel free to refactor the code into functions if it helps (if you have to modify the code), because I've found that it really helps clean up the code.
• line 187 has meta.append(df) even though meta seems to be undefined