Allow computing pose/twist from position/velocity

[efernandez]

This introduces the following changes:

1. Always read wheel joint position and velocity, simplifying the previous logic.
2. If wheel position/velocity is used to either compute the pose or twist, check for NaNs (this moves the previous check for NaNs).
3. Add the pose_from_joint_position (default: True) and twist_from_joint_position (default: False) params, which replace the position_feedback one.
4. Rename v_ variables into dp_ when their meaning is "difference of position" (or incremental position) and not velocity.

[efernandez]

Now that @p6chen 's #17 is merged, do you have any comments on this PR?

@servos @paulbovbel @afakihcpr

[efernandez]

I'm thinking of moving this (the reading) to the very top of the update method, so the values are readed the closest possible to the time the control cycle starts.

Now there's no problem reading before setting the dynamic params.

[paulbovbel]

Nice, LGTM

[afakihcpr]

LGTM

[efernandez]

Thanks.

Note that I've just push a fix to the velocity feedback test, which now uses the param pose_from_joint_position instead of the old position_feedback one.

Merging...