Better buffer motions and visual debug guide for free motion planning

[yijiangh]

This PR includes the following:

• previous/next movement-independent buffering linear motions for helping the free motion planner get out of narrow passages, fix #107 , #100 .
• Fixed a bug that prevents our MP from using our custom joint limits (specified in integral_timber_joints.planning.robot_steup.get_gantry_robot_custom_limits), now MP can finally sample in the right range. Before it was sampling around the full range, wasting efforts.
• Different MP algorithms can be specified by --mp_algorithm rrt or --mp_algorithm prm
• Now you can visualize the MP algorithm exploring the space live by: p -m integral_timber_joints.planning.run --design_dir 210419_AnticlasticShelter --problem shelter_process.json --movement_id A51_M1 --mp_algorithm birrt -v --debug --diagnosis --watch --draw_mp_exploration
• You can also force birrt to alternate between the two trees for exploration by adding birrt_enforce_alternate

[yck011522]

This is amazing~ Thanks for all the new improvements!