Add ability to bound contours

[gdt]

With an L-shaped irregular grid, such as in front of and on one side of a building, the plugin creates contours that extend through the building. Of course it does...

It would be great if one could also select a polygon that would bound the generated contours. Perhaps one should just intersect that polygon with the output layer, but perhaps communicating that the are where it makes sense to interpret data is limited is better.