Fix append mode double attributes

[franzpoeschel]

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When appending to an existing group/variable-based Series, don't define top-level attributes again (except basePath and openPMD, these are needed internally and ADIOS2 doesn't care as long as they are duplicate but equivalent) This fixes an Append mode bug that is already present on dev and has a low chance of making CI runs fail.

• [x] Merge #1218 first
• [x] Add CHECK_FILE task for other backends too (maybe not for ADIOS1)
• [x] ~~maybe merge CHECK_FILE and CREATE_FILE tasks~~, no this would require executing CREATE_FILE very early
• [x] cleanup
• [x] Support file creation via Append mode in group-based iteration encoding