Space simplifications, plus print_tree() signal for use with python

[wdeconinck]

• More simplifications concerning Space:
  • removed function "Space::nb_states()" --> use space.shape_function().nb_nodes()
  • removed function "Space::indexes_for_elements(elem_idx) --> use space.connectivity()[elem_idx]
• Full documentation of Space, containing why it is there, and how it is useful.
• Update of function Component::tree() component.tree( basic_mode , depth )
  • when basic_mode = true, only components marked basic will be printed
  • depth sets a maximum recursion level.
  • default value for basic_mode=false for unchanged behavior. (preferably default value for basic_mode=true upon consensus) default value for depth prints full tree
• print_tree() signal for use in python (useless for GUI): root.print_tree(basic_mode=True, depth=4)