The SetModelBounds method was not checking to see if xMin <= xMax,
etc. If incorrect bounds were specified, the resulting image data
contained unexpected values.

This patch checks for valid model bounds and generates a vtkError if
the bounds are incorrect.

Also added a unit test for vtkSampleFunction. The unit test provides
over 90% code coverage.

Change-Id: I9d36d316611a5c82c4f7212d73f4b9a7f7ab7965