ENH: expose control truncation of ndimage.gaussian_filter().

This change gives the user control over the size at which the Gaussian filter is truncated,
with default behavior to truncate at 4 standard deviations (the previous fixed value).
ENH: Pass extra keyword arguments to gaussian_laplace() and gaussian_gradient_magnitude() to gaussian_filter()

This change allows control over the truncation radius of the gaussian filter used in
gaussian_laplace() and gaussian_gradient_magnitude().
TST: ndimage: add more tests in test_gaussian_truncate()
Merge pull request #2767 from WarrenWeckesser/thouis-gaussian-filter-more-tests

ENH: ndimage: gaussian filter truncation