SeeYou: verify always waypoint altitude against ground altitude when terrain maps are loaded

[alus-]

It is handy to do always this check.

• Avoid this check for waypoints of the type "generic".
• For waypoints with a set altitude less than 5 m accept a maximum absolute delta of 10 m respect terrain altitude
• For waypoints with a set altitude greater than 5 m accept a maximum absolute delta of 15 m respect terrain altitude
  If the user don't want that he/she should simply not load any terrain altitude map.