Separated tfunc views from tfunc models

`SplineControl` and `StackedBarsControl` now contain the d3 view logic and inherit from `SVGRenderer`.

`SplineTransferFunction` and `StackedBarsTransferFunction` now contain the data models and sampling functions.

I tried to mirror the `ModelPool.js` structure in `TransferFunctions.js` for initializing the models and view-controllers. 

Future related todos:
* allow `StackedBarsControl` to be updated via Firebase like `SplineControl`
* change `StackedBarsControl` model to be rectangle heights (as a percentage) instead of rectangle top and bottom positions
* allow `StackedBarsControl` to use a logarithmic scale

resolves https://github.com/simtable/st-api/issues/6