Collective metadata

[1uc]

This PR enables users to configure collective metadata reads and writes (at file level). The heavy lifting was done in #620; here we simply restructure the proposed solution.

In more detail, this PR adds:

• ability to request collective MPI-IO metadata reads
• ability to request collective MPI-IO metadata writes
• explains the caveats of collective metadata reads
• add collective MPI-IO metadata writes to MPIOFileDriver.

[1uc]

I think it would be nice to have an example showing how to set everything up to use collectives, always. However this requires both this PR and #623. Therefore, we'll do this later.

[roblatham00]

sigh... there are probably some users out there somewhere that want collective write but independent read, or vice versa.

[1uc]

In which case, with property list plist, they would ask for collective reading:

plist.add(MPIOCollectiveMetadataRead{});

or collective writing:

plist.add(MPIOCollectiveMetadataWrite{});

The third option MPIOCollectiveMetadata reduces the typing effort and increases discoverability of the correct settings for the case where all operations are collective.

[codecov-commenter]

Codecov Report

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