BMI for Octave

[mcflugen]

We should define a standard BMI for Octave and include it here along with an example. Lucky for us, @RolfHut and @wknoben have already done this for MARRMoT. 🎉

[mdpiper]

See also #18.

[RolfHut]

Please add Niels Drost or other eWaterCycle members on this as well. Rolf