[Do not merge yet] Fix: Read ParticlePatches Attr

[ax3l]

ParticlePatches are no openPMD records but a special group. It does not need required record components such as unitDimension.

Ref.: https://github.com/openPMD/openPMD-standard/blob/1.1.0/STANDARD.md#sub-group-for-each-particle-species

Fix #1609

Update: oh, but we write

If the particlePatches sub-group exists, the following records within it are required and the entries in each record are stored in a per particle patch order:

Action Items

• either clarify standard or
• update openPMD-validator and openPMD-example data set

I will probably to the latter.

[ax3l]

Fix coming via openPMD/openPMD-validator#75 and openPMD/openPMD-example-datasets#23