Make SPs component endogenously find start_year and end_year

[lrennels]

Right now you need to set start_year and end_year for this component, but these will not be altered if, for example, you change the time dimension of the model. This was done because the init function needs access to this information but only has access to init(p,v,d) but not t. We may be able to update this within Mimi (see mimiframework/Mimi.jl#948) or do it in a backdoor way within the component.