[WIP] Try to fix const iterating

[Randl]

Added test to indicate the problem, attempted to fix.
@SylvainCorlay @JohanMabille ideas?

[SylvainCorlay]

The error that you are getting is that since the view is not const, it is calling the non-const overload of the function.

I have not tried yet, but I think that this example should build correctly if the returned type of view has type const xview instead of xview.

[SylvainCorlay]

At first glance, a solution might be to change https://github.com/QuantStack/xtensor/blob/master/include/xtensor/xview.hpp#L735 so that view type is a const xview when E is const.

E.g. using a std::conditional_t with std::is_const.

[SylvainCorlay]

cc @JohanMabille