Add `aggregation` keyword argument to `plot_demand` and `plot_demand!`

[yasirroni]

plot_demand and plot_demand! currently accept aggregate based on the doc, but didn't actually use it. Instead, it support aggregation with types Union{Type{ACBus}, Type{System}, Type{<:StaticLoad}, Type{<:AggregationTopology}}.

But, does this project will still be used when PowerAnalytics handle aggregation?

Related to: #100