ScheduledDiagnostics are not correctly deduplicated

[Sbozzolo]

ClimaDiagnostics is supposed to de-deuplicate diagnostics, but this is not always working correctly. The reason for this is that ClimaDiagnostics uses unique to identify duplicates. In turn, unique uses == to determine equality. Since we don't implement == for ScheduledDiagnostics, this defaults to ===, which does not what we want for RefValues.