Use schema-based versioning in ADIOS2 backend

[franzpoeschel]

This steals and isolates some commits of #855:

• Use YYYYMMDD format for the schema used in ADIOS2
• Automatically recognize the right schema when reading

TODO

• Remove some unnecessary changes that are only relevant to Variable-Based iteration layout #855
• Documentation
• Remove the new_attribute_layout from some leftover places
• Merge ADIOS2: v2.7.0+ #927 first
• Merge Decay ReadWrite mode into adios2::Mode::Read if the file exists #943 first
• This PR also fixes two bugs that I encountered while working on it. Should I separate those into another PR?
  1. Read-Write mode should inspect whether the file already exists and chose read-only or write-only mode accordingly. Properly supporting ADIOS Append mode is a Todo for the future though.
  2. Parallel IO tests check the wrong file name for existence.
• Rename __is_boolean__/(.*) to __openPMD_internal/is_boolean/\1 in new schema

[ax3l]

@franzpoeschel This PR also fixes two bugs that I encountered while working on it. Should I separate those into another PR?

Yes, that would be great in case I need to patch & cherry-pick them around! Thanks!

[franzpoeschel]

Ready for review (if CI goes green) :) @ax3l

[ax3l]

🎉