feat: Support reading Picard-style metrics files with comments above the header

[msto]

It would be helpful if Metric.read could parse metrics files with comments above the header, such as those produced by Picard. I've updated Metric.read to use the read_header() method introduced in #124 , and also simplified the typing on io.to_reader() and io.to_writer() (per Clint's suggestion)

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note I want to keep this separate from the MetricWriter feature branch, but it's dependent on the Metric._read_header method introduced in that branch.

I am waiting to merge until #123 is merged.