code: public static double trace( DMatrix5x5 a ) {
          return a.a11 + a.a22 + a.a33 + a.a44 + a.a55;
      }
docs: <p>
  This computes the trace of the matrix:<br>
  <br>
  trace = &sum;<sub>i=1:n</sub> { a<sub>ii</sub> }
  </p>
  <p>
  The trace is only defined for square matrices.
  </p>
  
  @param a A square matrix.  Not modified.