Relax thetaMode

[ax3l]

We want to relax the thetaMode geometry index order.

We want to continue to enforce that m is the slowest varying index in memory space, but r and z index order can be relaxed and derived from axisLabels.

WarpX order is different from FBPIC:

• RZ + openPMD : don't transpose data, save data with shape (Nm, Nz, Nr) BLAST-WarpX/warpx#3030

openPMD-viewer: generalization of data processing

• allow files of shape (Nm, Nz, Nr) or (Nm, Nr, Nz) openPMD-viewer#337