Fix `compare_values` for arrays with `NaN`, add deserialization for more union types

[GabrielKS]

This solves two issues:

1. Now that PiecewiseStepData HDF-serializes to an array that includes NaN, we need to allow NaN to be equal to itself when checking whether two things are the same — this is accomplished by changing == to isequal.
2. The new PowerSystems ThermalGenerationCost has a field of type Union{StartUpStages, Float64}, where StartUpStages is a NamedTuple. We didn't have code to deserialize that, but we did have code to deserialize Union{NamedTuple, Nothing}, so I generalized it.

I would have liked to have done most of the logic involved in (2) using the type system, but I did not seem to be able to do so without angering detect_unbound_args. For instance, this method solves the problem (though less generally) but is said to have unbound type parameters:

deserialize(::Type{Union{A, B}}, data::Dict) where {A <: _NOT_FROM_DICT, B <: NamedTuple} = deserialize(B, data)

I'm not sure I believe detect_unbound_args that there are unbound type parameters here, but I have opted for a more manual, less elegant, possibly more flexible approach.

[codecov[bot]]

Codecov Report

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

Project coverage is 75.82%. Comparing base (c723a2b) to head (ce66a4f). Report is 7 commits behind head on main.

Additional details and impacted files

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