- Added passive forces plugins

- Added new `cable` composite type:
  * The `initial` parameter specifies the joint at the starting boundary: `free`, `ball`, or `none`.
  * The boundary bodies are exposed with the names:`B_left` and `B_right`.
  * The vertex initial positions can be specified directly in the XML with the parameter `vertex`.
  * The orientation of the body frame **is** the orientation of the material frame of the curve.

- Added new `cable` passive force plugin:
  * Twist and bending stiffness can be set separately with the parameters `twist` and `bend`.
  * The stress-free configuration can be set to be the initial one or flat with the flag `flat`.
  * New cable example showing the formation of plectoneme.
  * New coil example.
  * New belt example showing interaction between twist and anisotropy.
  * Added test using cantilever exact solution.

PiperOrigin-RevId: 480033694
Change-Id: I491271bce8fccb185961477e903e5a72d172c8a3

src/user/user_util.h

@@ -124,6 +124,19 @@ void mjuu_offcenter(double* res, const double mass, const double* vec);
 // compute viscosity coefficients from mass and inertia
 void mjuu_visccoef(double* visccoef, double mass, const double* inertia, double scl=1);
 
+// update moving frame along a discrete curve or initialize it, returns edge length
+//   inputs:
+//     normal    - normal vector computed by a previous call to the function
+//     edge      - edge vector (non-unit tangent vector)
+//     tprv      - unit tangent vector of previous body
+//     tnxt      - unit tangent vector of next body
+//     first     - 1 if the frame requires initialization
+//   outputs:
+//     quat      - frame orientation
+//     normal    - unit normal vector
+double mju_updateFrame(double quat[4], double normal[3], const double edge[3],
+                       const double tprv[3], const double tnxt[3], int first);
+
 // strip path from filename
 std::string mjuu_strippath(std::string filename);