make bank mobilization factor an adjustable param

[jtgilbert]

can be calculated or estimated for individual rivers so it should be made a parameter. Should also include the script for calibrating the bank erosion equation so that it can be adjusted for different basins more easily.

[jtgilbert]

Actually, I should probably just add documentation about how to change the hard coding for this along with the bank erosion equation since it would need calibrated separately for any individual basin.