Fixes local_state_space_iteration_3 and adds a test comparing the various pruning routines

Fixes local_state_space_iteration_3 and adds a test comparing the various pruning routines used in dynare up to order 3. The considered pruning routines can be found in

• simult_.m
• local_state_space_iteration_2 MEX
• local_state_space_iteration_3 MEX
• pruned_state_space_system.m

I wrote a note to clarify the elements we shared in Mattermost Town Square channel about pruning. In short, every thing is fine in dynare's pruning, except local_state_space_iteration_3, which I fix here: KKSS and AVR are equivalent at second order if proper summation is enforced in AVR. See KKSS_AVR_equivalence.pdf for more details.