adds tools for the analysis of reduced diagnostics data

[aeriforme]

This PR adds some python modules to simplify the reading and plotting of reduced diagnostics data.

For now it works

• in 3D Cartesian geometry upon merging of PR #3751
• for ParticleEnergy, ParticleMomentum, ParticleNumber, FieldEnergy, FieldMaximum reduced diags.

The directory WarpX/Examples/Tests/reduced_diags_tools contains examples of usage of these tools.

A list of open issues to address follows (and more to be found):

• [ ] geometry:
  • [x] 1D Cartesian
  • [x] 2D Cartesian
  • [ ] RZ
• [ ] RhoMaximum diag does not include photon species and one cannot deduce if a species (given its name) is a photon species only from reduced diagnostics data
• [ ] ParticleExtrema diag does not include information on the species: maybe standardize name of diagnostic as ParticleExtrema_<species_names>
• [ ] integrate the tools in the python workflow
• [ ] documentation (separate page in postprocessing section)

[dpgrote]

Thanks for this PR! This will be very useful to have. I have some comments aimed toward improving the code.