Document openPMD particle "columns" if read back to a dataframe

[ax3l]

In our default reading of openPMD beam monitor data, we usually show how to load the beam data into a Pandas dataframe.

# initial/final beam
series = io.Series("diags/openPMD/monitor.h5", io.Access.read_only)
last_step = list(series.iterations)[-1]
initial = series.iterations[1].particles["beam"].to_df()
final = series.iterations[last_step].particles["beam"].to_df()

We need to extend the data analysis documentation to explain what units users can expect in those.