Masking (ndarray[nd])

[henon]

Doesn't work in ndarray[nd]?

System.NotSupportedException: Specified method is not supported.
   at NumSharp.NDArray.set_Item(Object[] indices_or_slices, NDArray value) in D:\SciSharp\NumSharp\src\NumSharp.Core\Selection\NDArray.Indexing.cs:line 202

Originally posted by @Oceania2018 in https://github.com/SciSharp/NumSharp/issues/359#issuecomment-541355337

[henon]

we need this for it: #365