Fix error in get_ps_freq_axis(), with outdate frame key

[FedeMPouzols]

Fixes #299

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[FedeMPouzols]

From the errors in the Mac builds, I see that this recent change in the schema is not yet in the "preconverted" and "S3" datasets, which I had missed. For now I've changed some of the checks that I added in this PR to be done only if not preconverted/s3.