Extend combine_channels support for more EPD datasets (STEP, EPT L3)

[jgieseler]

combine_channels function to combine fluxes of adjacent energy channels of EPT or HET has been added with v0.3.2 (for level-2 data). In principle, this would be nice to have for STEP and EPT level-3 data as well.

For STEP, the problem is that a lot of channel energy ranges overlap:

• protons/ions
  • old data: all!
  • new data: 1st 24
• electrons
  • old data:
    • per pixel: none
    • averaged: 1st 9
  • new data: 1st 3

For EPT L3, electron and proton data are in the same Dataframe, and the energy metadata is slightly different (mean, delta_plus & delta_minus instead of low_energy & bins_width), so there are some adjustments needed (e.g., the particle type needs then to be defined when calling combine_channels). Done

[jgieseler]

EPT L3 support development started with fb7140c. For now, the function recognizes the data and raises an exception.

[jgieseler]

EPT L3 support finished with 87a9506