Revise docstrings and repr functionality

This MR includes the following changes

• bumped minimum Python version to 3.9 and updated the Docker image. This was an unintended side effect of the removal of jinja2 were version changes in of packages used for building the documentation and testing which required a more comprehensive update.
• documentation
  • extensive revision of the docstrings with respect to content, style, and formatting
  • small reorganization of the documentation with core classes and functions now collected under a separate tab in the "Function reference" section
  • updated and cleaned up notebooks on cluster expansion training under "Advanced topics" section of documentation
  • dropped cloud_sptheme.ext.table_styling. This package, which has not seen updates in a very long time, slightly improved table formatting but locked jinja to an outdated version.
• changes to API
  • added _repr_html_ methods for key objects for rendering HTML views in jupyter notebooks
  • added some new __str__ methods
  • added ClusterExpansion.cutoffs and ClusterExpansion.chemical_species as properties (forwarded as copies from underlying ClusterSpace object)
  • renamed ClusterSpace.orbit_data to ClusterSpace.as_list (property)
  • added ClusterSpace.to_dataframe method
  • added ClusterExpansion.to_dataframe method
  • added StructureContainer.to_dataframe() method
  • renamed get_number_of_orbits_by_order to number_of_orbits_by_order and changed it to property
  • removed print_overview functions (role taken by as/to_dataframe and __str__ methods)

closes #617