too many dependecy for EnergyHistogramData data reader

[PrometheusPi]

If loading the python module EnergyHistogramData, shipped with picongpu, via:

from picongpu.plugins.data import EnergyHistogramData

the module imageio (perhaps also pandas?) is required. Since this is only a data reader, no image processing library should be required. Perhaps this is a simple error in include order since the visualization module:

from picongpu.plugins.plot_mpl import EnergyHistogramMPL

To avoid conflicts for new users, this could be fixed - but in my opinion has a low priority.

EDIT:
The error is more severe - loading EnergyHistogramData loads all other modules and thus all other dependencies (not needed for this simple module) need to be installed.

/.../picongpu/lib/python/picongpu/plugins/data/__init__.py in <module>
      2 from .phase_space import PhaseSpaceData
      3 from .png import PNGData
----> 4 from .radiation import RadiationData
      5 from .sliceFieldReader import FieldSliceData
      6 from .emittance import EmittanceData

I would consider this a bug - I will have to cross check this against the most up to date dev.

[PrometheusPi]

@psychocoderHPC might need a back-port to the release candidate

[PrometheusPi]

The issue still exists after @s9105947 restructuring