Allow tendon springlength attribute to take two values between which the spring produces no force (a deadband).

If given two non-decreasing values, `springlength` specifies a deadband for spring stiffness, inside of which the force is 0 and outside of which force behaves like a regular spring, with the setpoint corresponding to the nearest value. This can be used to create tendons whose limits are enforced by springs rather than constraints, which are cheaper and easier to analyse. See test/engine/testdata/tendon_springlength.xml example model.

Spring-limited spatial tendons whose lower range value is 0 are rendered as catenaries.

Fixes #520.

PiperOrigin-RevId: 484509706
Change-Id: I6698e94ee36168f52d501e83057559805e05172d

src/xml/xml_util.cc

@@ -565,7 +565,7 @@ string mjXUtil::FindValue(const mjMap* map, int mapsz, int value) {
 // read attribute "attr" of element "elem"
 //  "len" is the number of floats or doubles to be read
 //  the content is returned in "text", the numeric data in "data"
-//  return true if attribute found, false if not found and not required
+//  return number of elements found
 template<typename T>
 int mjXUtil::ReadAttr(XMLElement* elem, const char* attr, const int len,
                       T* data, string& text, bool required, bool exact) {