AP_AHRS: add a singleton-getter cast into AP_AHRS_NavEKF

This will allow libraries which absolutely require the EKF to easily get
a cast copy of the singleton.

libraries/AP_AHRS/AP_AHRS.h

@@ -686,4 +686,10 @@ private:
 
 namespace AP {
     AP_AHRS &ahrs();
+
+    // use ahrs_navekf() only where the AHRS interface doesn't expose the
+    // functionality you require:
+#if AP_AHRS_NAVEKF_AVAILABLE
+    AP_AHRS_NavEKF &ahrs_navekf();
+#endif
 };

libraries/AP_AHRS/AP_AHRS_NavEKF.cpp

@@ -1642,5 +1642,11 @@ uint8_t AP_AHRS_NavEKF::get_primary_gyro_index(void) const
     return AP::ins().get_primary_gyro();
 }
 
+
+AP_AHRS_NavEKF &AP::ahrs_navekf()
+{
+    return static_cast<AP_AHRS_NavEKF&>(*AP_AHRS::get_singleton());
+}
+
 #endif // AP_AHRS_NAVEKF_AVAILABLE