Relaxed mode: warn and ignore invalid?

[ax3l]

Regarding:

• ADIOS Particles are not fully openPMD-compatible ComputationalRadiationPhysics/picongpu#3119 (comment)
• Allow user to store non-openPMD information openPMD-standard#115

It might be easier to use the API if we try to skip and verbosely warn when invalid formatting of records are detected, instead of erroring out on the whole series.

Nevertheless, recovery from such situations needs extra logic and plumbing.