v0.21

Changes:

• Blocks programming with Google Blockly introduced.
• Official Clover simulation config added.
• roswww_static added: static version of roswww (served with Monkey).
• simple_offboard: detect killswitch when auto_arm is set
• simple_offboard: bug fixes.
• genmap.py: Add x0 and y0 shift for markers coordinates
• led.launch and main_camera.launch minor changes.
• Other improvements and fixes.