H5Easy: Support creating extensible datasets with Eigen::ArrayXXd

[nickanthony-dgl]

I am trying to create a dataset that can later be resized like so:

Eigen::ArrayXXd data = generateData();
auto dset = H5Easy::dump(f,"/myData", data, std::vector<size_t>{0, 0}, dataOptions); // Initialize an extensible dataset

I get the following error message:

...\include\highfive\h5easy_bits\H5Easy_public.hpp(109): error C2039: 'dump_extend': is not a member of 'H5Easy::detail::io_impl<Eigen::ArrayXXd,void>'
  ...\include\highfive\h5easy_bits/H5Easy_Eigen.hpp(22): note: see declaration of 'H5Easy::detail::io_impl<Eigen::ArrayXXd,void>'
  ...\include\highfive\h5easy_bits/H5Easy_public.hpp(109): note: the template instantiation context (the oldest one first) is
  myproject/mysource.cpp(139): note: see reference to function template instantiation 'HighFive::DataSet H5Easy::dump<Eigen::ArrayXXd>(HighFive::File &,const std::string &,const T &,const std::vector<size_t,std::allocator<size_t>> &,const H5Easy::DumpOptions &)' being compiled
          with
          [
              T=Eigen::ArrayXXd
          ]
...\include\highfive\h5easy_bits\H5Easy_public.hpp(109): error C3861: 'dump_extend': identifier not found

Possibly related to #1018