Write model inputs to log file

[sicasley]

• Maybe add a new function to do this...
• Could use the model_building_arguments.txt file (output from ModelMap::model.build) to extract parameter values and write them to a log file.
• This log file could then be used to re-run a particular model by referencing input parameters from the log.
• Model runs should append to log file, so it maintains a record of historical runs.