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.