Requiring a period here forces signal models to place a time frame on
when their signal is valid. This also allows consumers of signals to
have some expectation of when a prediction should come to fruition.
The risk management model is intended to check the algorithm's positions
at the end of each time step to potentially exit positions that are losing
too much.
This change includes a check to prevent users from overriding methods required
by the framework. This is non-ideal and we should perhaps look into alternatives
to this approach, which could involve additional methods on IAlgorithm. In order
to not lose access to these events at the algorithm level, we could expose them
as C# events (not sure python compatibility?)
Michael Handschuh