change where function to return vector of vectors

[wolfv]

the

where(a > 3)

should return a vector of vectors, or actually a vector of xtensor<std::size_t, 1>, like in numpy.

[wolfv]

Moved to 0.17.0.

We need to carefully think about this change. Also, when we have the "new" dynamic view from @JohanMabille we can use it in combination with islice to recreate index_view in the same way it exists in numpy.