dump_npz does not work with bool array

[raphaelsulzer]

Trying to dump a boolean array leads to the error below.
The code compiles fine if I replace the bool with int.
Is this a bug or am I doing something wrong?

vector<bool> bv = {true,false,true};
auto xbv = xt::adapt(bv);
xt::dump_npz("test.npz","test",xbv,false,false);

leads to the following error:

In file included from /usr/local/include/xtensor/xarray.hpp:20:0,
                 from /usr/local/include/xtensor/xadapt.hpp:20,
                 from /usr/local/include/xtensor/xnpy.hpp:34,
                 from /usr/local/include/xtensor-io/xnpz.hpp:30,
                 from /home/adminlocal/PhD/cpp/mesh-tools/src/IO/fileIO.h:36,
                 from /home/adminlocal/PhD/cpp/mesh-tools/src/IO/fileIO.cpp:5:
/usr/local/include/xtensor/xcontainer.hpp: In instantiation of ‘const value_type* xt::xcontainer<D>::data() const [with D = xt::xtensor_adaptor<std::vector<bool>&, 1, (xt::layout_type)1, xt::xtensor_expression_tag>; xt::xcontainer<D>::value_type = bool]’:
/usr/local/include/xtensor/xnpy.hpp:655:26:   required from ‘void xt::detail::dump_npy_stream(O&, const xt::xexpression<E2>&) [with O = xt::detail::binary_vector; E = xt::xtensor_adaptor<std::vector<bool>&, 1, (xt::layout_type)1, xt::xtensor_expression_tag>]’
/usr/local/include/xtensor-io/xnpz.hpp:480:32:   required from ‘void xt::dump_npz(std::__cxx11::string, std::__cxx11::string, const xt::xexpression<D>&, bool, bool) [with E = xt::xtensor_adaptor<std::vector<bool>&, 1, (xt::layout_type)1, xt::xtensor_expression_tag>; std::__cxx11::string = std::__cxx11::basic_string<char>]’
/home/adminlocal/PhD/cpp/mesh-tools/src/IO/fileIO.cpp:774:51:   required from here
/usr/local/include/xtensor/xcontainer.hpp:578:31: error: passing ‘const storage_type {aka const std::vector<bool>}’ as ‘this’ argument discards qualifiers [-fpermissive]
         return storage().data();
                               ^
In file included from /usr/include/c++/7/vector:65:0,
                 from /usr/include/boost/property_map/vector_property_map.hpp:15,
                 from /usr/include/boost/property_map/property_map.hpp:601,
                 from /usr/local/include/CGAL/property_map.h:19,
                 from /home/adminlocal/PhD/cpp/mesh-tools/src/base/cgal_typedefs.h:6,
                 from /home/adminlocal/PhD/cpp/mesh-tools/src/IO/fileIO.cpp:1:
/usr/include/c++/7/bits/stl_bvector.h:920:5: note:   in call to ‘void std::vector<bool, _Alloc>::data() [with _Alloc = std::allocator<bool>]’
     data() _GLIBCXX_NOEXCEPT { }
     ^~~~
In file included from /usr/local/include/xtensor/xarray.hpp:20:0,
                 from /usr/local/include/xtensor/xadapt.hpp:20,
                 from /usr/local/include/xtensor/xnpy.hpp:34,
                 from /usr/local/include/xtensor-io/xnpz.hpp:30,
                 from /home/adminlocal/PhD/cpp/mesh-tools/src/IO/fileIO.h:36,
                 from /home/adminlocal/PhD/cpp/mesh-tools/src/IO/fileIO.cpp:5:
/usr/local/include/xtensor/xcontainer.hpp:578:31: error: void value not ignored as it ought to be
         return storage().data();
                               ^