`plot_powerdata!` broken on `combine_categories=false`

[Gabriel Konar-Steenberg]

When plot_powerdata! is called with the default combine_categories=true, it uses PA.combine_categories to get a single DataFrame to pass to plot_dataframe!. When combined_categories=false, it tries to pass the to plot_dataframe! entire PA.PowerData.data, which is of type Dict{Symbol, DataFrame} and thus of course doesn't work. It's not clear what the desired behavior should be. Make one plot_dataframe! plot per item in the dictionary?

A principled solution to this issue will likely be wrapped up in my PowerAnalytics redesign project.