categorize_data misses slack variables

[josephmckinsey]

https://github.com/NREL-Sienna/PowerAnalytics.jl/blob/42ba52e9d044e62330a037d6044542992282a6de/src/get_data.jl#L605

Because of module difficulties, the categorization can miss unserved energy since it doesn't look for "PowerSimulations.SystemBalanceSlackUp"