Adios2 warn groupbased encoding

[franzpoeschel]

Should not go into 0.15 release cycle since it advises to use variable-based encoding

• Merge Warn on BP5+Blosc in ADIOS2 v2.9 up to patch level 1 #1497 first
• Add some environment variable or similar to deactivate this The warning is now only printed for the new ADIOS2 encoding (group table), so this does not affect legacy workflows
• Maybe adapt tests Postponed to when variable-based encoding becomes more useful
• Entirely forbid usage without steps? Would make things easier
  EDIT: adios2.usesteps option is now removed, is automatically used if WriteIterations is used in the frontend
• For a less abrupt transition, apply warnings only in new schema (i.e. group table enabled)

More context: #1457

The warning is printed only when using the (currently opt-in) group table feature, for the sake of a smoother transition for users.
We should soon make group tables opt-out. A good time would be when we have better support for variable-based encoding.

Breaking because: It's no longer possible to open ADIOS2 group-based files with linear access mode if they don't use the group table feature.

[ax3l]

@franzpoeschel ready for rebase :)

[franzpoeschel]

@ax3l You recently mentioned a workflow that you have where you repeatedly close and reopen an Iteration to write data to it. Since this workflow implies that you cannot use variable-based encoding, I assume that you use file-based encoding instead?
I want to avoid discontinuing group-based encoding in ADIOS2 while there are workflows which actually use it.

[franzpoeschel]

I would like to adapt all examples and tests to this change. Since this requires more features for file-based enconding (e.g. reopening a closed iteration) and for variable-based encoding (e.g. random-access) alike, I will postpone this PR for now.