Implement parflow.Run object to build control file

[arezaii]

The current tcl_builder implementation needs to be replaced by the parflow.Run object in python PFTools.

• subset_conus.py implements parflow.Run to build run script.
• remove builders/tcl.py from codebase
• remove builders.tcl.py from docs
• parflow.Run builds a simple script to simulate a rainfall event on a parking lot domain setup