Transfer function

Currently doing interpolation with a lookup table. Turns out it's hard to solve for the interpolated curve exactly. Eventually we should probably make the lookup table an actual lookup table, instead of an array that you search through!

In Ecostem.js, I added a commented example of how to use the object:

````
var elevationToDroplets = new TransferFunction([0, 200], 'm', [0, 400], 'droplets / m^2', 'Rainfall vs. elevation');
var numMeters = 150;
var numDroplets = transferFunc(numMeters);
````

Might be time to think about where to put a menu for editing model variables (i.e. opening up a transfer function, or some other interface).