[wpilib] Add function to adjust LQR controller gain for pure time delay (#2878)

There were three options for where to put this function:

1. A free function in LinearQuadraticRegulator.h. Returning a K matrix
   means the user can't use the LinearQuadraticRegulator in a loop
   anymore.
2. A default argument added to ctors in LinearQuadraticRegulator for a
   time delay (default of 0). This has the smallest API footprint from
   the user perspective, but it bloats the already substantial
   constructor overload set of LinearQuadraticRegulator.
3. A member function in LinearQuadraticRegulator that modifies the
   internal K. This would still have to take in a LinearSystem or (A, B)
   pair because the ctor doesn't store it. Storing it internally feels
   like paying for what we don't use most of the time.

I went with option 3.

I verified the tests's expected values in Python with
scipy.linalg.fractional_matrix_power().

Closes #2877.

wpimath/src/main/java/edu/wpi/first/wpiutil/math/Matrix.java

@@ -368,6 +368,25 @@ public class Matrix<R extends Num, C extends Num> {
     return toReturn;
   }
 
+  /**
+   * Computes the matrix power using Eigen's solver.
+   * This method only works for square matrices, and will
+   * otherwise throw an {@link MatrixDimensionException}.
+   *
+   * @param exponent The exponent.
+   * @return The exponential of A.
+   */
+  public final Matrix<R, C> pow(double exponent) {
+    if (this.getNumRows() != this.getNumCols()) {
+      throw new MatrixDimensionException("Non-square matrices cannot be raised to a power! "
+            + "This matrix is " + this.getNumRows() + " x " + this.getNumCols());
+    }
+    Matrix<R, C> toReturn = new Matrix<>(new SimpleMatrix(this.getNumRows(), this.getNumCols()));
+    WPIMathJNI.pow(this.m_storage.getDDRM().getData(), this.getNumRows(), exponent,
+          toReturn.m_storage.getDDRM().getData());
+    return toReturn;
+  }
+
   /**
    * Returns the determinant of this matrix.
    *