apstools SpecWriterCallback metadata plugin

Looks for #MD control line control lines. These lines contain metadata supplied to the bluesky RunEngine and recorded during the execution of a scan. The data are stored in a dictionary of each scan: scan.MD. If there are no #MD control lines, then scan.MD does not exist.

see https://prjemian.github.io/spec2nexus/source/_filewriters.html#apstools.filewriters.SpecWriterCallback

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