ENH: make scripts executable and add shebang

apps/drs_spot67_import.py

+#!/usr/bin/env python3
 import argparse
 from scenes import save_scenes
 from scenes.spot import get_spot67_scenes

apps/s2_download.py

+#!/usr/bin/env python3
 import argparse
 from scenes import TheiaDownloader, utils
 import datetime

apps/s2_import.py

+#!/usr/bin/env python3
 import argparse
 from scenes import save_scenes
 from scenes.sentinel import get_local_scenes

apps/search.py

+#!/usr/bin/env python3
 import argparse
 from scenes import load_scenes, Index, utils