- 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

python/mujoco/functions.cc

@@ -65,7 +65,7 @@ PYBIND11_MODULE(_functions, pymodule) {
   DEF_WITH_OMITTED_PY_ARGS(traits::mj_printSchema,
                            "filename", "buffer", "buffer_sz")(
       pymodule, [](bool flg_html, bool flg_pad) {
-        constexpr int kBufferSize = 30000;
+        constexpr int kBufferSize = 40000;
         auto buffer = std::unique_ptr<char[]>(new char[kBufferSize]);
         const int out_length = InterceptMjErrors(::mj_printSchema)(
             nullptr, buffer.get(), kBufferSize, flg_html, flg_pad);