dWMax is Required if plasticityFlag is True

Hello everyone. I just made a commit that changes dWMax to a required parameter if plasticityFlag is true. (If plasticityFlag is false and dWMax is defined in the params file, it will not be read and a warning will be issued that the parameter was skipped.)

Also, initialize_base now initializes dWMax to NaN. This is to catch any circumstances where the dWMax member variable is used but the dWMax parameter is not read — this should not happen.