[Cleanup] Refactor MeshLevel to Accommodate Mesh/Particle Discretization

[povolny1]

Describe the issue With the merge of #1756 into develop, a ParticleManager will be registered on any MeshLevel. Conceptually, a given MeshLevel corresponds to a given discretization (they will be renamed DiscretizationLevel or something similar soon), and so it does not make sense for a MeshLevel to contain both mesh and particle managers.

Proposed cleanup In a discussion with @rrsettgast, @CusiniM, @TotoGaz and @homel1, it was proposed to create a base class for mesh/discretization levels and derive from it MeshDiscretization and ParticleDiscretization classes. These will only register their respective managers, removing the issue.