jni_ vec_ mean
fun jni_vec_mean(env: CPointer<JNIEnvVar>, thisObject: jobject, a: jdoubleArray, aOffset: jint, aSize: jint): Double
fun jni_vec_mean(env: CPointer<JNIEnvVar>, thisObject: jobject, a: jdoubleArray, aOffset: jint, aSize: jint): Double