Fix openPMD Meta-Data

[ax3l]

Tests

Check created files with openPMD-validator to conform with the openPMD-standard.

Fixes

Add the positionOffset record of openPMD, which can be used for coarse particle positions or general offsets of a particle species.
This refers now to the offset added by the reference particle.

Follow-up to #299

[ax3l]

@cemitch99 here for positionOffset and in position, do we want to output s+delta s instead of ct + c*dt?

[cemitch99]

Not sure I follow what the intended output is here. I understand that by "ct" you mean "ref_part.t", but what is meant by "dt"? Does this represent one or more particle values of the "t" coordinate?

In general, I would say that "s" is qualitatively different from the other variables like ct. It is an independent variable, it is associated only with the reference particle, and the other (beam particle) values are given as functions of "s".

[ax3l]

Thanks, ah yes I was confused for a second... So per particle, splitting the longitudinal position (for a fixed s) into ref_part.t (should we call this ref_part.ct?) and position.ct makes sense...

[cemitch99]

Yes, that's true.

[ax3l]

Ok, will do in #350

[cemitch99]

It looks ok to me, thanks.