include writing/updating of models in portalcast

[juniperlsimonis]

for example, if a user inputs controls that would point a model to a different data set, that should be acknowledged by the functions, which means the model script needs to be re-written
however, we don't want to automatically write all models named in models, as we don't want it overwriting any hand-made models.
i think the best option is then to have it update models that are in controls_models, which means we need a new function update_models that is v similar to, but distinct from fill_models

[juniperlsimonis]

done in #141