- 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

test/xml/xml_native_writer_test.cc

@@ -848,7 +848,8 @@ TEST_F(XMLWriterTest, WriteReadCompare) {
         std::string xml = p.path().string();
 
         // if file is meant to fail, skip it
-        if (absl::StrContains(p.path().string(), "malformed_")) {
+        if (absl::StrContains(p.path().string(), "malformed_") ||
+            absl::StrContains(p.path().string(), "plugin")) {
           continue;
         }