Add support for arbitrary default node lookup name

Makes it possible to apply modifiers to elements even if the name doesn't match.
This is necessary to support default values for tendons.

unity/Runtime/Importer/MjXmlModifiers.cs

@@ -28,12 +28,17 @@ namespace Mujoco {
       _root = root;
     }
 
-    public void ApplyModifiersToElement(XmlElement element) {
+    public void ApplyModifiersToElement(XmlElement element, string elementName=null) {
+      // Allow overriding the element name for defaults lookup, needed for tendon
+      if (elementName == null) {
+        elementName = element.Name;
+      }
+
       // Combine all defaults into one. At this stage, we want to overwrite attributes defined by
       // the previous defaults.
       var aggregateDefaults = _root.CreateElement("aggregate");
       // Root default leaf should be processed only once, and handled first (so it's overriden).
-      var rootDefaultLeaf = _root.SelectSingleNode($"/mujoco/default/{element.Name}") as XmlElement;
+      var rootDefaultLeaf = _root.SelectSingleNode($"/mujoco/default/{elementName}") as XmlElement;
       if (rootDefaultLeaf != null) {
         CopyAttributes(rootDefaultLeaf, aggregateDefaults);
       }
@@ -43,7 +48,7 @@ namespace Mujoco {
         var defaultClassElement =
             _root.SelectSingleNode($"descendant::default[@class='{className}']") as XmlElement;
         // Ancestry iterates up in the tree, but we want to apply changes from remote to specific.
-        var ancestors = GetDefaultAncestry(defaultClassElement, element.Name).Reverse();
+        var ancestors = GetDefaultAncestry(defaultClassElement, elementName).Reverse();
         foreach (var defaultAncestor in ancestors) {
           CopyAttributesOverwriteExisting(defaultAncestor, aggregateDefaults);
         }