Revert default output precision type for Contour filter points

The default was changed to floats a while back to handle an apparent regression when the input points type was integral (it happens), but this wasn't great when the input point precision was double. This commit changes the default value back to vtkAlgorithm::DEFAULT_PRECISION, which sets the precision of the output to the precision of the input.

The output precision type remains exposed for corner cases where the the input precision type is an integral type. Such cases have come up on the mailing list more than once, and the result is a truncation of the contour coordinates to integral values, leading to cube-ish contours.