coyote: CGKML_GROUNDOVERLAY__DEFINE

Description
This program implements the KML GroundOverlay element. For 
reference, see  the `Google KML Reference Documentation `.
A GroundOverlay element draws an image overlay draped onto the terrain
of a Google Earth display.
Categories
Graphics, FileIO
Examples
See the `cgKML_File` object for examples of how to create a KML file.
Author
FANNING SOFTWARE CONSULTING::
   David W. Fanning 
   1645 Sheely Drive
   Fort Collins, CO 80526 USA
   Phone: 970-221-0438
   E-mail: david@idlcoyote.com
   Coyote's Guide to IDL Programming: http://www.idlcoyote.com
History
Change History::
   Written, 28 October 2012 by David W. Fanning.
Copyright
Copyright (c) 2012, Fanning Software Consulting, Inc.
e purpose of this method is to establish an abstract KML GroundOverlay class. For reference, see
tps://developers.google.com/kml/documentation/kmlreference?hl=fr#groundoverlay.
Keywords
altitude: in, optional, type=double
    Specifies the distance above the earth's surface, in meters, and is interpreted 
    according to the altitude mode
altmode: in, optional, type=string, default='clampToGround'
    The altitude mode. Possible values are "clampToGround", which ignores the `Altitude`
    value and drapes the overlay over the terrain. The other possible mode is "absolute",
    which sets the altitude of the overlay relative to sea level, regardless of the 
    actual elevation of the terrain beneath the element. For example, if you set the 
    altitude of an overlay to 10 meters with an absolute altitude mode, the overlay will 
    appear to be at ground level if the terrain beneath is also 10 meters above sea level. 
    If the terrain is 3 meters above sea level, the overlay will appear elevated above the 
    terrain by 7 meters.
latlonbox: in, optional, type=fltarr
    A four-element float array that specifies the top, bottom, right, and left sides
    of a bounding box that the ground overlay is aligned to. The elements of the array
    also correspond to [north, south, east, west] coordinates, which should be specified
    in decimal degrees. North/South coordinates are in the range -90 to +90, and East/West
    coordinates are in the range -180 to 180.
latlonquad: in, optional, type=fltarr
    An 8-element array describing the four corners of a quadrilateral defining the overlay area. 
    Exactly four coordinate pairs have to be provided, each consisting of floating point values 
    for longitude and latitude. The coordinates must be specified in counter-clockwise order 
    with the first coordinate corresponding to the lower-left corner of the overlayed image. 
    The shape described by these corners must be convex.
rotation: in, optional, type=float, default=0.0
     Specifies a rotation of the overlay about its center, in degrees. Values can be ±180. 
     The default is 0 (north). Rotations are specified in a counterclockwise direction.
     Used only with the `LatLonBox` array.
_ref_extra: out, optional
    Any keywords for the superclass objects are allowed.
e purpose of this method is to destroy anything we created that may leak
mory or need to be released from use.
e cgKML_GroundOverlay class definition module. It is a container object 
d represents part of the KML file hierarchy.
Params
class: out, optional, type=structure
    The class definition returned as a structure variable. Occassionally useful.