coyote: CGKML_OVERLAY__DEFINE

Description
This program implements the KML abstract OVERLAY class. For 
reference, see  the `Google KML Reference Documentation `.
An Overlay object is a basic KML class, and should be inherited by,
for example, a cgKML_GroundOverlay object, which extends and implements 
this KML abstract class.
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 Overlay class. For reference, see
tps://developers.google.com/kml/documentation/kmlreference?hl=fr#overlay.
Keywords
color: in, optional, type=string
    Color values are expressed in hexadecimal notation, including opacity (alpha) values. 
    The order of expression is alpha, blue, green, red (aabbggrr). The range of values for 
    any one color is 0 to 255 (00 to ff). For opacity, 00 is fully transparent and ff is 
    fully opaque. For example, if you want to apply a blue color with 50 percent opacity 
    to an overlay, you would specify the following: COLOR = "7fff0000".
draworder: in, optional, type=integer
    This element defines the stacking order for the images in overlapping overlays. 
    Overlays with higher values are drawn on top of overlays with lower values.
href: in, optional, type=string
    A URL that identifies the location of the image associated with this Overlay. The
    location can be either a local file (e.g., 'myimage.png') or a URL to an image
    on a web server (e.g., 'http://www.idlcoyote.com/images/myimage.png').
_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_Overlay class definition module. This is an abstract class
at will be inherited by overlay objects (GroundOverlay, and ScreenOverlay, among others).
sically, any KML element that should be added to a KML file will inherit this
ject. It is a container object and represents part of the KML file hierarchy.
Params
class: out, optional, type=structure
    The class definition returned as a structure variable. Occassionally useful.