[BUG] Prophet model doesn't use upper/lower constraints

[cdeterman]

It seems best to have a separate issue to address this problem. The underlying prophet model is not able to do the min and max capacity constraints. The following reproduces the problem regardless of whether the underlying data is transformed.

reg = HTSRegressor(model='prophet', capacity_min=0, capacity_max=10000)
reg.fit(df, nodes=hier)
reg.predict(steps_ahead=30)

Not sure if there would be a way to either modify the internal generation of the future object or if can be passed by the user.