Inpainting nan elements in 3-d

An extension of inpaint_nans to 3 dimensions

Every once in a while I get asked for a 3-d version of inpaint_nans, so I modified my existing 2-d code to run in 3 dimensions. This code has only two methods as options. I picked what I felt were the two most useful methods from inpaint_nans to extend to 3-d.

See the included published demos for examples of use.