[COLLECTION]: Add code examples to API

[larshinueber]

Hi all,

This issue is a collection of functions for which small code examples should be added to the docstrings.
Feel free to comment functions where you feel this is needed or edit this top-level comment directly.
If any function needs to be exposed to Python or missing docstrings entirely, add them to #211.

tudatpy.numerical_simulation:

• environment
  • AtmosphereModel
• environment_setup
  • gravity_field.ring_gravity
  • gravity_field.polyhedron_from_mu
  • gravity_field.polyhedron_from_density
  • rotation_model.constant_rotation_model Example is misleading
• propagation_setup
  • step_size_control_blockwise_matrix_tolerance
  • simulator.propagation_results.single_arc_results
  • MultiArcPropagatorSettings.processing_settings
  • HybridArcPropagatorSettings.processing_settings
  • SingleArcPropagatorSettings.processing_settings
  • SingleArcPropagatorSettings.termination_settings
  • SingleArcPropagatorSettings.print_settings
  • propagator.custom_state
• estimation
  • see Add code examples to estimation api docs #237
• propagation
  • SingleArcSimulationResults.processed_state_ids
  • SingleArcSimulationResults.propagated_state_ids