free_agency_rd

Things still to do

1. convert all code/contract_offer/build_model_POSITION.py files to code/contract_decision/build_model_WR.py format
2. move those files to code/contract_decision folder
3. fold those files into contract_main.py (should just be a couple lines importing the file and then calling the function like I did for WRs)
4. continue to improve UI