Add new model `VlasovAmpereOneSpecies`

Solves the following issue(s):

Closes #260 (closed)

Core changes:

• Add method show_accumulated_spline_field() to AccumulatorVector for quick sanity check; also disable check_rhs for the moment.

Model-specific changes:

• New kinetic model VlasovAmpereOneSpecies has been added. It consists of the two propagators PushEta and VlasovAmpere. The latter propagator was formally called VlasovMaxwell, but was now renamed.
• Renamed kinetic model VlasovMaxwell to VlasovMaxwellOneSpecies
• Commented model VlasovMasslessElectrons (not working)
• Rename epsilon_unit to epsilon (same for alpha)

Documentation changes:

• Improved, very detailed docstrings of the models VlasovAmpereOneSpecies and VlasovMaxwellOneSpecies. These should be used as good examples for other model's docstrings.

Other:

• Added parameters and diagnostics for linear Landau damping with VlasovAmpereOneSpecies to struphy-simulations.