Add capabilities to restrict/weight specific joints

[sea-bass]

It's great that there are joint centering / joint displacement cost functions available, but what if you want finer-grained control for specific joints? Two examples would be:

• Suppose you want to limit some joints to move as little as possible compared to other joints, so relative weighting.
• Suppose you want a specific joint value to fall within a custom range that is separate from the joint center/limits, so specifying other bounds.

I actually think that a tutorial on how to add such a cost function to pick_ik with something like this would be super helpful.

[sea-bass]

@Robotawi this could be a good idea for a custom cost function tutorial.

[Robotawi]

Noted, @sea-bass!