S2_download() enhancements

• tries parameter added allowing automatic error handling
• skipExisting parameter is tri-state now:
  • always downloads the data only if local copy doesn't exist (no matter its size)
  • samesize downlads the data if local copy doesn't exist or has different size
  • never downloads the data no matter if local copy exists
• timeout now defaults to 1800 (seconds)

0.4.0

See NEWS.md