WIP: nearest neighbor lookup selection

[davidbrochart]

I'm just trying to see if I'm at least going in the right direction, so please ignore the hacks for now (global variable...).

The following code:

#define XFRAME_DEFAULT_LABEL_LIST xtl::mpl::vector<double, int, std::size_t, char, XFRAME_STRING_LABEL>

double global_tolerance = 0.;

#include "xframe/xvariable.hpp"
#include "xframe/xvariable_view.hpp"

int main(int argc, char* argv[])
{
    using coordinate_type = xf::xcoordinate<xf::fstring>;
    using variable_type = xf::xvariable<double, coordinate_type>;
    
    xt::xarray<double> data
          {{1., 2., 3.},
           {4., 5., 6.},
           {7., 8., 9.}};
    
    auto lat = xf::axis({0., -0.1, -0.2});
    auto lon = xf::axis({0., 0.1, 0.2});
    
    auto var = variable_type(
        data,
        {
            {"lat", lat},
            {"lon", lon}
        }
    );

    std::cout << var << std::endl;

    std::cout << xf::select(var, {{"lat", xf::range(0., -0.1)}, {"lon", xf::range(0.11, 0.19)}}, 0.02) << std::endl;

    return 0;
}

Produces the following output:

{{1, 2, 3},
 {4, 5, 6},
 {7, 8, 9}}
Coordinates:
lat: (0, -0.1, -0.2, )
lon: (0, 0.1, 0.2, )

(2, 3, )
(5, 6, )
Coordinates:
lat: (0, -0.1, )
lon: (0.1, 0.2, )