using reducers on masked_view

[ThibHlln]

Hi,

There seems to be the same kind of compilation error as in #1981 with reducer functions (e.g. xt::mean).

#include <xtensor/xtensor.hpp>
#include <xtensor/xmath.hpp>
#include <xtensor/xio.hpp>

int main(int argc, char* argv[])
{
    xt::xtensor<double, 2> t
            {{ 5.3, 4.2, 5.7, 2.3 },
             { 5.3, 4.2, 5.7, 2.3 },
             { 5.3, 4.2, 5.7, 2.3 }};

    xt::xtensor<bool, 2> mask
            {{ true, true, false, true },
             { true, true, true, true },
             { true, true, true, false }};

    // compilation error
    auto v = xt::masked_view(t, mask);
    std::cout << xt::mean(v, 1) << std::endl;

    // workaround
    auto w = xt::where(mask, t, NAN);
    std::cout << xt::nanmean(w, 1) << std::endl;
}

So, I am wondering whether reducers are meant to work on xt::masked_view?

Being able to do it would be quite useful to avoid unnecessary copies (as in my workaround I think).

Thanks.