[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/native/cpp/jni/WPIMathJNI.cpp

@@ -105,6 +105,28 @@ Java_edu_wpi_first_math_WPIMathJNI_exp
   env->SetDoubleArrayRegion(dst, 0, rows * rows, Aexp.data());
 }
 
+/*
+ * Class:     edu_wpi_first_math_WPIMathJNI
+ * Method:    pow
+ * Signature: ([DID[D)V
+ */
+JNIEXPORT void JNICALL
+Java_edu_wpi_first_math_WPIMathJNI_pow
+  (JNIEnv* env, jclass, jdoubleArray src, jint rows, jdouble exponent,
+   jdoubleArray dst)
+{
+  jdouble* arrayBody = env->GetDoubleArrayElements(src, nullptr);
+
+  Eigen::Map<
+      Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor>>
+      Amat{arrayBody, rows, rows};
+  Eigen::Matrix<double, Eigen::Dynamic, Eigen::Dynamic, Eigen::RowMajor> Apow =
+      Amat.pow(exponent);
+
+  env->ReleaseDoubleArrayElements(src, arrayBody, 0);
+  env->SetDoubleArrayRegion(dst, 0, rows * rows, Apow.data());
+}
+
 /*
  * Class:     edu_wpi_first_math_WPIMathJNI
  * Method:    isStabilizable