Seperable filters should be treated as separable

[leios]

The Gaussian blur filter (for example), can be changed to be a blur along x, transpose, then blur along y (and transpose again). This might be more efficient in the end.

It's best if users give just a 1D filter and say it's separable. We can then call a separate separable_filter(...) function, which does the filter!(...), transpose(), and filter!(...), like we do for sobel.

note, this is related to #38, which introduces filtering.