Add script to retrieve Jpp headers

scripts/get_jpp.sh

+#!/usr/bin/env bash
+if [ ! -d 'jpp' ]; then
+    echo "Cloning Jpp..."
+    git clone --depth 1 https://$KM3NET_USERNAME:$KM3NET_PASSWORD@git.km3net.de/common/jpp.git
+else
+    echo "Fetching latest master"
+    cd jpp
+    git pull
+fi