SWW header modification

[ole]

Tony Hunter has requested the following to be added to the sww file headers to enable more automated harvesting of metadata from the bathymetry files used in modelling runs.

• the geographic bounding box co-ordinates to be output as 4 tuplets in an ascii format (priority highest)
• as a secondary request the existing header information and the geographic bounding box to be output in XML format. (priority middle)

First request is the highest priority as it will lead to testing of harvesting capabilities for Inundation Impact model runs, into MITS.