Add support for writing data-less sectors in IMD.

[tdaede]

This adds sector record 0 type support to the IMD writer.

It currently adds IDAM areas into the sector list. Most writers
seem to be able to skip these appropriately, however I'm not
sure mixing data types in the sector list is the best approach.
Other approaches included a None or special DAM, or a dedicated
list of IDAMs like the IAM list.

The reader can read these back but currently does not encode a
flux stream without a DAM.

(it does seem this feature is relatively poorly supported in
IMD readers as well)

[keirf]

Yes this is a tough one and I'm sure mixing types in the list will break some list users. Mypy ought to be able to detect that but I probably nobbled it with the not-very-nice assignment to iam vs sectors in decode_raw via a generic List[Any]. I need to think about a simpler representation of sectors in their variety of damaged forms. Good call though on handling "missing data", so I will get this merged in somehow.