Add BepiColombo/SIXS-P L3 support to SERPENTINE tools

[jgieseler]

With https://github.com/serpentine-h2020/SEPpy/releases/tag/v0.3.0, BepiColombo/SIXS-P level 3 data can be loaded. Now it's need to be added to the tools.

• Dropdown menu
• Onset notebook
• TSA notebook (needs some work in functions calculate_particle_speeds and get_channel_energy_values)
• Data loading notebook
• Dynamic spectrum notebook (not straightforward with non-boxcar, unconnected energy channels)

Note that the internal-only level 2 data are (at least partially) already included, but they have a different structure.

[jgieseler]

The checked changes are already included in the development version of SEPpy and this repo.

• When SEPpy gets a new release version, (done: https://github.com/serpentine-h2020/SEPpy/releases/tag/v0.4.0)
• this needs to be updated on the JupyterHub,
• and afterwards the changes in this repo (Update dropdown menus with data levels and BepiColombo/SIXS-P #72) can be merged.