DotSpatial Tools, Allow setting of processing extent for raster tools

twhitvine[CodePlex]
Raster analysis such as interpolation from point data is currently restricted to the bounding box of the points. It'd be nice if the analysis extent could be set to the extent of a watershed, for example.

Consider this scenario. I search for data in a watershed. I download data from sites found WITHIN the watershed. I interpolate to raster using the IDW tool. The result does not cover the full extent of the watershed, because all points used in the interpolation
were in the interior of the watershed.

Start by locating the IDW raster tool and checking how its analysis extent is currently derived from the input points. Define how callers can provide a larger extent, such as a watershed boundary, and verify that interpolation covers that extent rather than only the points' bounding box.

Written by the indexing model from the issue text.