[wpimath] Make Java DCMotor API consistent with C++ and fix motor calcs (#3046)

The stall torque, stall current, and free current are now multiplied by
the number of motors instead of just the stall torque. This produces the
same values for Kt and Kv regardless of the number of motors; the motor
resistance still affects the system response.

For an elevator model, the response should be the same as before since a
factor of "number of motors" shows up in the same place in the
acceleration calculation, but the current calculation will also be
correct now.

wpimath/src/main/native/include/frc/system/plant/DCMotor.h

@@ -55,12 +55,12 @@ class DCMotor {
                     units::radians_per_second_t freeSpeed, int numMotors = 1)
       : nominalVoltage(nominalVoltage),
         stallTorque(stallTorque * numMotors),
-        stallCurrent(stallCurrent),
-        freeCurrent(freeCurrent),
+        stallCurrent(stallCurrent * numMotors),
+        freeCurrent(freeCurrent * numMotors),
         freeSpeed(freeSpeed),
-        R(nominalVoltage / stallCurrent),
-        Kv(freeSpeed / (nominalVoltage - R * freeCurrent)),
-        Kt(stallTorque * numMotors / stallCurrent) {}
+        R(nominalVoltage / this->stallCurrent),
+        Kv(freeSpeed / (nominalVoltage - R * this->freeCurrent)),
+        Kt(this->stallTorque / this->stallCurrent) {}
 
   /**
    * Returns current drawn by motor with given speed and input voltage.