Arduino Mega base support

This is a very basic basament case for arduino mega2560, there are:
- pins to fix the arduino into
- border openings for the usb and power supply
- border low profile to keep pin headers labels visible

- if want to increase basament thickness open freecad, select Extrude001 part and change extrusion length to something else than actual -1.2mm ; then issue CTRL+R to apply change and export Fillet003 ( topmost part ) as STL