Screw mobilizer should allow runtime pitch change & similarly for other elements

[sherm1]

The pitch should be stored in an instance variable in the State so that it can be changed without requiring topology to be rebuilt. (Gazebo has an API for setting it.)

EDIT: there are many other elements in Simbody that need state-modifiable versions of their parameters. Here is the beginning of a checklist:

• Screw mobilizer's pitch
• PointInPlane constraint's normal direction, height, and follower point location
• Ball constraint's two point locations (and radius?)
• lots more