ADIOS2: New Layout in openPMD 2

[ax3l]

This is a meta-issue to document new layout options in ADIOS2.

Related to openPMD/openPMD-standard#221

• mutable attributes: Use ADIOS variables for openPMD attributes Use ADIOS variables for openPMD attributes #813
• OPENPMD_NEW_ATTRIBUTE_LAYOUT: generalize control variable to "openPMD2_adios2_schema" ("new_layout")?
• store the YYYYMMDD as attribute of the new layout so we can identify it if we need to in reads. just bump during dev whenever we change things
• variable encoding: add a __steps__ variable in base path Variable-Based iteration layout #855
• decide on bool support for variables: ADIOS2 Bool: Dataset Support #921