Elaborate Type Hierarchy for Key Types

[GabrielKS]

Currently, VariableType, ParameterType, etc. have no type hierarchy. In new PowerAnalytics, we define some unions:

"The various key entry types that can work with a System"
const SystemEntryType = Union{
    PSI.VariableType,
    PSI.ExpressionType,
}

"The various key entry types that can be used to make a PSI.OptimizationContainerKey"
const EntryType = Union{
    SystemEntryType,
    PSI.ParameterType,
    PSI.AuxVariableType,
    PSI.InitialConditionType,
}

to handle these types more generically. @daniel-thom and I think something like this (not wedded to this layout, but something that groups these key types in useful ways) would be good to include in the type hierarchy itself.