Support parametrized optional FCS keywords

[zbjornson]

Adds support for setting $Pn[DFLOPTV] and $PnCALIBRATION. Skips the deprecated keywords ($Gn*, $PKn, $PKNn) and the rarely used ones ($RnI, $RnW, $CSVnFLAG).

FlowKit test run: https://github.com/primitybio/FlowIO/actions/runs/3130423878

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Last PR for now. There are a few other changes I'd consider, but none are pressing:

• Don't upper-case all keys. Some importers are erroneously case-sensitive, so this can break those. Mixed case can also be easier to read as a human (things like P2DetectorName).
• Use a different delimiter than / since that's also found in file paths and the source of many invalid FCS files.
• Allow zero-length files. They're valid and sometimes useful.