Performance improvements to FK

[PeterMitrano]

This is a pretty big PR but the main focus is on improving speed. Other changes:

• Chain.forward_kinematics no longer allows you to supply a dict with only a subset of joint values, since this was deemed confusing and ambiguous
• Chain.forward_kinematics now allows you to pass in indices for which you want (not string names). You can convert between indices and strings using frame_to_idx or idx_to_frame, or Chain.get_frame_names
• Joint limits are now parsed and stored if the urdf/sdf/xml has them.
• "_frame" is no longer appended to everything. This used to be inconsistent between mjcf and urdf/sdf parsers, now it is consistent.
• several new units tests (adding test with a mujoco model of Val)

[PeterMitrano]

actually JK this isn't ready