This links together the generic MFD and the NavDisplay; it allows a fairly easy method to add a Map page to an MFD device.
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See: http://wiki.flightgear.org/Canvas_MFD_Framework
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Instantiate parameters:
1. pfd_device (instance of PFD_Device)
2. instrument display ident (e.g. mfd-map, or mfd-map-left mfd-map-right for multiple displays) - used to map to the property tree
3. layer_id: main layer in the SVG
4. nd_group_ident : group (usually within the main layer) to place the NavDisplay
5. [optional] switches - used to connect the property tree to the nav display. see the canvas nav display.
To add a canvas nav display page simply do
me.some_page = PFD_NavDisplay.new(me.PFD,"Situation", "mpcd-sit", "ID", "jtids_main");
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