Geometry: Cylindrical

[ax3l]

Let's formalize the m=0 special case of thetaMode geometry for cylindrical geometry.

Refs.:

• Add support for fields with cylindrical geometry in DataReader openPMD-viewer#305
• Change geometry of fields to thetaMode in openPMD output AngelFP/Wake-T#18

[AngelFP]

Hi Axel,
Thanks for starting this discussion already! Similarly to the thetaMode geometry, and following the style in STANDARD.md, we could define it as follows:

cylindrical: regularly-spaced mesh in the r-z plane. The mesh arrays are stored as a two-dimensional record where the first and second axis correspond, respectively, to the r and z directions.