Rename some (should be) private attributes/parameters/methods of the Network class

[fwitte]

Specifically these are

• set_defaults: _set_defaults
• init_only
  • just rename flag: presolve_only
  • add explicit method: nw.presolve("design", init_path="...")
• _init_set_properties: something like _impose_SI_values_for_specified_parameters
• _init_properties: _guess_starting_values
• _init_design/_init_offdesign: _prepare_...
• init_val0: _guess_connection_starting_values
• solve_determination: _check_determination
• solve_loop: _solve_loop
• solve_control: _solve_iteration
• postprocessing: _postprocess
• process_components: _postprocess_components
• process_connections: _postprocess_connections
• solve_equations: _solve_equations
• check_variable_bounds: _adapt_to_variable_bounds