Group Based Encoding Causes Issues with BP5

[ax3l]

See:

• Reading a BP5 series (groupBased) using 120+ GB memory, any fix around it? openPMD/openPMD-api#1724

• impactx/src/elements/diagnostics/BeamMonitor.H

  Line 84 in 875cb37

  BeamMonitor (std::string series_name, std::string backend="default", std::string encoding="g", int period_sample_intervals=1);

We probably need to work on closing steps, changing encoding defaults, etc.

[ax3l]

For BP5, we definitely have to disable g, e.g., openPMD/openPMD-api#1457 (comment)