Add terrain contours to map

[srmainwaring]

Use data available in the terrain module to overlay terrain contours on the map.

Motivation

• Be able to overlay terrain contour lines on the existing slip map.
• Use existing terrain data sources to match information available on the FC.
• Initial step in adding support to mavproxy for configuring off-board terrain planning for planes. The idea is to replace the dependency on rviz and simplify the user workflow the example used here https://discuss.ardupilot.org/t/offboard-terrain-navigation-for-plane-with-ros-2-ap-dds/114418.

Use

Select the number of contour levels, grid resolution and grid extent using the map settings:

STABILIZE> map set
STABILIZE>           
          brightness 1
    circle_linewidth 1
 contour_grid_extent 20000.0
contour_grid_spacing 30.0
      contour_levels 20
           font_size 0.5
        loitercircle False
         rallycircle False
     setpos_accuracy 50
        showahrs2pos 0
        showahrs3pos 0
         showahrspos 1
       showclicktime 2
       showdirection False
         showgps2pos 1
          showgpspos 1
          showsimpos 0
           showwpnum True

The terrain resolution is determined by the source used. The default is SRTM3 which may be switched before loading the contours using:

STABILIZE> terrain set source SRTM1

To show the contours use the context menu:

Hide the contours (but do not remove - saves recalculating when showing again):

To select a different region, remove then show again:

Tasks

• add menu item to toggle contour visibility
• add settings to control number of levels (and/or spacing)

Follow up / optional

• add settings to control the contour colour map
• add contour labels
• replace dependency on matplotlib.pyplot.contour with ContourPy
• automatically update / calculate contours as the map is moved / zoomed as it's currently fixed