Make `add_stable_points` optional in `IndShockConsumer`

[Mv77]

The solver for our most widely used class IndShockConsumerType calculates various points of theoretical interest via its method add_stable_points.

https://github.com/econ-ark/HARK/blob/7ce7138b6d9617a28fd4448936be3d61acad21d8/HARK/ConsumptionSaving/ConsIndShockModel.py#L1078

@mnwhite recently pointed out that this method can take up to 70% of the total time that it takes to solve a model.

These points are often not needed and, as user, I would like to have the option to not try to find them if I am doing structural estimation or calculating a Jacobian.

I want to propose that we disable the calculation of these points in the default solver, keeping them as an option to be activated by the user if he wants to calculate them.