File io: avizo file loader and numpy converter

avizo_io.py file loader tool fully functional for ASCII and BINARY file formats. Still requires work to enable loading of AmiraMesh-Zip file format data sets.
In particular, I need suggestions for tests to include in the test_avizo_io.py file.
Also, write functionality is still on the TODO list.
PEP8: 2 spaces between top-level functions