Plane: split up altitude handling

this creates altitude.pde for altitude handling, getting ready to add
terrain following
AP_Terrain: added some helper functions

this adds easier to use helper functions for terrain handling
AP_Terrain: fixed cm conversion error
AP_Common: added terrain_alt flag to Location

signifies that the altitude is a above terrain altitude
AP_Terrain: added location_to_relative_home() function
Plane: added TERRAIN_FOLLOW parameter

when enabled RTL, CRUISE and rally points will use terrain altitude