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.
It is better to do this with a context manager:

```
with open(..., 'r') as input_file:
    pass
```

The context manager will take care of closing the file when you are done and will guarantee that in _all_ cases the file will be properly closed