Added io readers/writers and plot3d (untested)

src/empyre/io/__init__.py

src/empyre/io/field_plugins/__init__.py

src/empyre/io/field_plugins/llg.py

src/empyre/io/field_plugins/numpy.py

src/empyre/io/field_plugins/ovf.py

src/empyre/io/field_plugins/tec.py

src/empyre/io/field_plugins/text.py

src/empyre/io/field_plugins/vtk.py

src/empyre/io/io_field.py

src/empyre/vis/plot3d.py