Model ReFlex pressure sensors in simulation

[Adam Conkey]

Right now in Gazebo simulation we ignore the TakkTile sensors for the reflex hand. The goal is to add at least basic contact sensing which can be achieved by Gazebo's contact sensor plugin. Better yet, we can enable full pressure sensing with their pressure sensor plugin but there is less documentation and examples for this so it may be a bit more work.

Mabel Zhang from Penn and now at OSRF was kind enough to provide her work on modeling boolean contact sensors on a beta version of the hand. This is attached below and we can use that as an example.

[Adam Conkey]

The contact sensors were added with this pull request on this package, and with this related pull request in ll4ma_robots_description.

[Adam Conkey]

Pressure sensors are added on a feature branch but more work is required to make them map reasonably to the real world. Right now they compute literal pressure (force/area) and since the sensors are so small in area, even small forces give huge pressures. Actual sensors range [0,255] so I think we can get a reasonable estimate just by hooking up a force sensor to the real finger and getting a calibration for rough division factor from mapping of normal forces to observed pressure.

Also want to add Gaussian noise so it's more realistic. Can even estimate sufficient statistics empirically from the real sensors from their baseline readings.

[Adam Conkey]

Note the pressure sensor changes to URDF in ll4ma_robots_description have been merged to main branch with this PR.