Add partial support for nifty images.
A plugin can have an open() overload to handle directy the filename as a backend may not support c++ streams. Fix issue for permissive load with non-compatible conversion. * mln/io/loader.hpp: Fix permissive load issue. * mln/io/nifti/imread.hpp, * mln/io/nifti/nifti_plugin.hpp: New. * mln/io/plugin.hpp: Add open overload and istream support traits.