AP_NavEKF3: derivation: make generation slightly easier

Add a shell script to run the generate_1.py file. Foresight tells us
that we will need multiple such files.

Also ignore the generated code files as they are already out of date and
we are aiming to document provenance and make them unnecessary anyway.

libraries/AP_NavEKF3/derivation/.gitignore

@@ -0,0 +1 @@
+generated/*.cpp

libraries/AP_NavEKF3/derivation/generate.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+cd "$(dirname "$0")"
+rm -rf generated # ensure generated directory exists and is empty
+mkdir -p generated
+
+./generate_1.py