Remodel gravity compensation

[stefanscherzinger]

Goal

Rework and possibly redesign the gravity compensation feature of the cartesian_force_controller.

Background

Gravity compensation is suitable when we work with tools whose mass cannot be neglected during operation. A possible use case is holding some heavy polishing tool that, upon reorientation, causes high force-torque measurements in the wrist-mounted sensor that corrupt the cartesian_force_controller's target wrench or distort the cartesian_compliance_controller's stiffness. Some rudimentary functionality is present, but this feature needs more documentation and tests.

High-level feature requests

• When specified adequately, mounted tools should appear mass-less for the controller
• Support both sensors that can be biased (to zero) and such that can't
• Allow to specify tool and gravity characteristics during runtime
• Guarantee plausible behavior, i.e. no controller jumps during biasing
• Test the functionality with integration tests.

Steps

• Implement a light-weight, simulated force-torque sensor for integration tests
• ...

Resolves #39