Migrate to SI units

[jyearsley]

Much of this code is still legacy from its development for the Columbia River TMDL. for example, energy budget units are kcal/sec/meter2, flows are in ft3/sec, distances are in feet/miles, etc. Ultimately, the code would be improved by unifying and making consistent the units. Also see related issue regarding hardwired constants.

[YifanCheng]

After discussion with John, to clarify this problem:

1. change the unit in RBM code to SI unit
2. Add more unit input options. If the input unit is not SI unit, we just need to do more unit conversion.