Add documentation to clump breakage functions in 'bf' module

This merge request covers the changes related to preparing documentation for the 'bf' module. The following major changes are introduced:

• examples with videos,
• 'bf' module description.

Also, some minor changes:

• moved the bf module from py/bf/ to py/,
• added a minor improvement (now, before erasing the sphere all its interactions are erased as well).