Particle Species: loadPositions

[ax3l]

The openPMD base standard describes the handling of position and positionOffset compositions for particle species.

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

For reading, we should implement this as a convenient member function, to avoid performing this well-defined behavior via multiple loadChunk() calls and additions on the user side.