modify format access to waypts via global list. (#478)

modify format access to waypts via global list. (#478)

As a rule writers should not modify waypoints on the global list.
One way this can happen is if the writer uses the global waypoint
list, global_waypoint_list.  Via the list pointers to non-const
Waypoints can be accessed.  The three writers that use the
global_waypoint_list all did not modify any waypoints, but it was not obvious.
This PR makes it obvious by getting const Waypoint pointers from the list.
This required some const correctness corrections in these writers.

As a rule readers should not modify waypoints they didn't add to the
global list.
One way this can happen is if the reader uses the global waypoint list.
One reader was inapproprieately modifying waypoints on the global list.
While this is OK for waypoints it added, it is not OK for any waypoints
that were added by other readers.  This PR fixes a bug where the
netsumbler reader, intending to modify waypoints it added to the list,
actually modified all waypoints on the list.