Add `FunctionData` Printing

[GabrielKS]

I implemented some more human-friendly printing of the various FunctionData:

These could be useful when I do the same thing for the PSY cost structs (https://github.com/NREL-Sienna/PowerSystems.jl/issues/1048).

Also fixed a typo in existing printing test code.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.53%. Comparing base (7fe2da2) to head (5c19680).

Additional details and impacted files

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[GabrielKS]

Above formats are unchanged, added a third format that is human-readable but less verbose, for use where FunctionData are nested within other types that will convey that information (see e.g. https://github.com/NREL-Sienna/PowerSystems.jl/pull/1125):

[GabrielKS]

Re-requesting review for this ^ compact addition