Extraction of a subset to another mth5 archive (export_subset)

[kkappler]

Context: sometimes my h5 files can get large and I would like to export a subset of the data in the MTH5 to a smaller file that I can share to another user, or transfer over network to another machine. For example, I have a 20GB file, but a few short runs <1GB that I want to share.

It would be nice if I could do something like:

m.open_mth5(file_name)
m.export_subset(other_file_name, dataset_df)
m.close_mth5()

Now I would have a separate mth5 called other_file_name.

It would be ideal if all the metadata cleanly transferred to the new file, and of the input dataframe looked like an mth5 channel summary.
In that case, I could just grab the m.channel_summary.to_dataframe(), then apply some queries to the df, reducing it to dataset_df and call a one liner.