[wpimath] Add lastValue() method to filters (#6351)

wpimath/src/main/native/include/frc/filter/LinearFilter.h

@@ -371,6 +371,13 @@ class LinearFilter {
     return retVal;
   }
 
+  /**
+   * Returns the last value calculated by the LinearFilter.
+   *
+   * @return The last value.
+   */
+  T LastValue() const { return m_outputs.front(); }
+
  private:
   wpi::circular_buffer<T> m_inputs;
   wpi::circular_buffer<T> m_outputs;

wpimath/src/main/native/include/frc/filter/MedianFilter.h

@@ -61,6 +61,13 @@ class MedianFilter {
     }
   }
 
+  /**
+   * Returns the last value calculated by the MedianFilter.
+   *
+   * @return The last value.
+   */
+  T LastValue() const { return m_valueBuffer.front(); }
+
   /**
    * Resets the filter, clearing the window of all elements.
    */

wpimath/src/main/native/include/frc/filter/SlewRateLimiter.h

@@ -75,6 +75,13 @@ class SlewRateLimiter {
     return m_prevVal;
   }
 
+  /**
+   * Returns the value last calculated by the SlewRateLimiter.
+   *
+   * @return The last value.
+   */
+  Unit_t LastValue() const { return m_prevVal; }
+
   /**
    * Resets the slew rate limiter to the specified value; ignores the rate limit
    * when doing so.