Openpmd-api: ADIOS2: closed attribute read warning

[ax3l]

Describe the bug
Reading back ADIOS2 data, I see warnings of the type:

[ADIOS2] Warning: Attribute with name __is_boolean__/data/0/closed has no type in backend.
[ADIOS2] Warning: Attribute with name __is_boolean__/data/100/closed has no type in backend.
[ADIOS2] Warning: Attribute with name __is_boolean__/data/500/closed has no type in backend.
[ADIOS2] Warning: Attribute with name __is_boolean__/data/300/closed has no type in backend.
[ADIOS2] Warning: Attribute with name __is_boolean__/data/200/closed has no type in backend.
[ADIOS2] Warning: Attribute with name __is_boolean__/data/400/closed has no type in backend.

in my logs.

To Reproduce

Read-back of an ADIOS2 BP4 file that closed iterations with ::close() during write.

Expected behavior

Maybe we can use an attribute type that we can re-identify easily (int) or fix read-back of our self-made bool types in ADIOS2?

Software Environment

• version of openPMD-api: 0.12.0
• installed openPMD-api via: conda-forge
• operating system: Ubuntu 18.04 (64bit) on x86
• machine: my laptop
• name and version of Python implementation: CPython 3.8.5
• version of HDF5: 1.10.6
• version of ADIOS1: 1.13.1
• version of ADIOS2: 2.6.0
• name and version of MPI: w/o MPI

Additional context

Saw this while trying to read-back ZFP compressed data sets.

[franzpoeschel]

Ah, I already fixed that same issue a while ago on my streaming branch, I can cherry-pick the commit quickly.