ADIOS2 variable shape LocalValue

[franzpoeschel]

Ref https://adios2.readthedocs.io/en/v2.9.2/components/components.html#shapes

This PR considers LocalValues in ADIOS2 merely an implementation detail, without actually exposing an API analogous to to how LocalValues work in ADIOS2.

The reasoning is:

1. LocalValues are useful not only for their API, but also since they give a hint to ADIOS2 that the variable should be handled as metadata and take part e.g. in metadata aggregation. With this PR, datasets such as the particlePatches in PIConGPU (one single value per MPI rank) can be given to the metadata system without changing a single line of code in PIConGPU. This can be crucial in streaming setups: When each reading rank loads the entire dataset naively (without this setting), the data plane of SST will open a connection between every writer and reader for just a single value each. Better to aggregate.
2. If needed at some point, a LocalValue-like API can still be exposed on top of this.

TODO

• Documentation