Remove splitting of sww files

[ole]

Now that large NetCDF format has been in place for a good while and the viewer has been update to use this, we can remove the need for splitting into chunks less than 2GB. I think this is in store_timestep()

I propose doing this in two steps:

1: Increase the variable controlling the splitting to a number large enough to prevent it. Verify all tests and validations work. 2: Carefully remove the splitting algorithm in store_timestep() and the codes that were modified to observe splitted sww files. Verify all tests and validations.

This supersedes ticket:257 which I have closed.

[ole]

Jane raised the valid issue that some parts of ANUGA (and indeed other software) may read entire sww files into memory and therefore rely on splitting.

The 'right' solution would be to rewrite any such application to use blocking. However, me may retain ANUGA's ability to do the splitting or alternatively provide a splitting tool as part of the package.