Export of results needs to be faster

[ole]

The function sww2dem is being used extensively to produced inundation maps. It is fairly slow since it has never been profiled and optimised. Ideas for optimising it include:

Specify polygon rather than rectangle and assign a value like -9999 to points outside of the polygon. Whether this will speed it up is unclear. Another option would be to profile the function and optimise the bottleneck if there is one.

The function needs some TLC in any case, as it has been evolving over the years without much attention.

Unfortunately, it may be that the slow performance comes from the interpolation code which is known to fairly slow.