[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/math/WPIMathJNI.java

@@ -69,11 +69,21 @@ public final class WPIMathJNI {
    * Computes the matrix exp.
    *
    * @param src  Array of elements of the matrix to be exponentiated.
-   * @param rows how many rows there are.
+   * @param rows How many rows there are.
    * @param dst  Array where the result will be stored.
    */
   public static native void exp(double[] src, int rows, double[] dst);
 
+  /**
+   * Computes the matrix pow.
+   *
+   * @param src      Array of elements of the matrix to be raised to a power.
+   * @param rows     How many rows there are.
+   * @param exponent The exponent.
+   * @param dst      Array where the result will be stored.
+   */
+  public static native void pow(double[] src, int rows, double exponent, double[] dst);
+
   /**
    * Returns true if (A, B) is a stabilizable pair.
    *