Remove monitor transforms

Recently implemented monitor transforms provided a flexible way to transform state variables of model output to expressions of interest, such as the LFP for the Epileptor model.

Experience using this has shown that it may be far more flexible than required, and a better alternative would be for models to define "output" variables which correspond to observables which we want to model.

Most monitors would be applied to observables, though some monitors like the Raw monitor, used for debugging, would apply to state variables.

Some UI & DB changes may be required.