Add mju_fill, mju_symmetrize and mju_eye utility functions.

PiperOrigin-RevId: 484226916
Change-Id: I79593d54e903ddacab0f859bfc6d17eccad7d3a1

doc/APIreference.rst

@@ -5909,6 +5909,17 @@ mju_zero
 
 Set res = 0.
 
+.. _mju_fill:
+
+mju_fill
+~~~~~~~~
+
+.. code-block:: C
+
+   void mju_fill(mjtNum* res, int n, mjtNum val);
+
+Set res = val.
+
 mju_copy
 ~~~~~~~~
 
@@ -6082,6 +6093,28 @@ mju_transpose
 
 Transpose matrix: res = mat'.
 
+.. _mju_symmetrize:
+
+mju_symmetrize
+~~~~~~~~~~~~~~
+
+.. code-block:: C
+
+   void mju_symmetrize(mjtNum* mat, int n);
+
+Symmetrize square matrix :math:`M = \frac{1}{2}(M + M^T)`.
+
+.. _mju_eye:
+
+mju_eye
+~~~~~~~
+
+.. code-block:: C
+
+   void mju_eye(mjtNum* mat, int n);
+
+Set mat to the identity matrix.
+
 .. _mju_mulMatMat:
 
 mju_mulMatMat