Add get_load_shed method to Analyze

[danielolsen]

Now that we have load_shed being calculated and extracted using REISE.jl, it would be nice to be able to query these results from the Scenario object. When REISE.jl with load shedding enabled has been used as the engine, it's as easy as grabbing the pickle files the same way we grab PG. Otherwise, we probably want to either return a (sparse) empty dataframe, or in the case of REISE with 5% load curtailment steps, we may want to return a sparse dataframe with entries for every load bus for the intervals with curtailment (when curtailment occurs, otherwise return sparse & empty).