AHRS: make P gain on PI roll/pitch controller tunable

this is to make it easier for Craig and Alan to tune

libraries/AP_AHRS/AP_AHRS_DCM.cpp

@@ -37,6 +37,7 @@ const AP_Param::GroupInfo AP_AHRS::var_info[] PROGMEM = {
 	// @Range: 0 .4
 	// @Increment: .01
     AP_GROUPINFO("YAW_P", 0, AP_AHRS_DCM, _kp_yaw),
+    AP_GROUPINFO("RP_P",  1, AP_AHRS_DCM, _kp),
     AP_GROUPEND
 };
 

libraries/AP_AHRS/AP_AHRS_DCM.h

@@ -23,7 +23,7 @@ public:
                 _ki = 0.0087;
 		_ki_yaw = 0.01;
 
-                _kp = 0.4;
+                _kp.set(0.4);
 		_kp_yaw.set(0.4);
 	}
 
@@ -44,9 +44,9 @@ public:
 
 	// settable parameters
 	AP_Float	_kp_yaw;
+	AP_Float	_kp;
 
 private:
-	float		_kp;
 	float		_ki;
 	float		_ki_yaw;
 	bool		_have_initial_yaw;