coyote: CGGOOGLE_METERSPERPIXEL

Description
This funtion returns the number of meters per pixel for a particular zoom level
in images returned using Google's Static Image API.
Categories
Utility
Params
zoomlevel: in, optional, type=integer, default=12
   The Google zoom level used in retrieving Google Maps using the static image API.
   Documentation: https://developers.google.com/maps/documentation/staticmaps/.
Examples
For example, to find the meters per pixel for a zoom level of 12::
   IDL> Print, cgGoogle_MetersPerPixel(12)
        38.218514
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
Written, 20 June 2012.
Updated the algorithm for determining meters per pixel to use WGS-84 equitorial radius
   based on suggestion by Alain LeCacheax. 11 Sept 2012. DWF.
Copyright
Copyright (c) 2012, Fanning Software Consulting, Inc.