Culverts: the ability to identify a single triangle instead of multiple triangles encompassed by the exchange polygon

[hudson]

From Rudy:

ALso..... another pressing issue I have with regard to testing the CULVERT routine, is the ability to identify a single triangle instead of multiple triangles encompassed by the exchange polygon

When the domain grid size is not fine enough the current routine, fails However it would be far better to identify a single triangle and use that triangle as the exchange area if the cell resolution is not high enough as in the above.

Making the cells as small as a sub metre diameter pipe..... slows the entire run down