Use new IS functionality to streamline `FunctionData` operations

[GabrielKS]

The point of Sienna-Platform/InfrastructureSystems.jl#492 was to provide nice, sugary implementations of common FunctionData operations. As we encounter them, we should replace ad hoc FunctionData computations in PSI with these standard implementations. Tagging @rodrigomha for visibility

[luke-kiernan]

The original idea was sound--there's significantly more function manipulation in PSI--but this issue doesn't apply to POM the same way. It'd just be a single-line name change in a handful of spots, not a meaningful simplification.