Allow species in paths, remove FieldMesh gridDataOrder

[ChristopherMayes]

Previously phase space data for a species was written directly in particlesPath. The standard dictates that these should be organized into names: https://github.com/openPMD/openPMD-standard/blob/upcoming-2.0.0/STANDARD.md#particle-records.

• ParticleGroup.write will now name this group by the species name
• ParticleGroup will now accept a single group name when loading. Multiple names is not implemented.
• The changes are backwards compatible with the old format.

Additionally, we remove gridDataOrder from FieldMesh array data, and instead use axisLabels to determine the multidimensional array ordering. The changes are backwards compatible with the old format.

[ken-lauer]

This should be merged before #68