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.
```
for header_line in header_list:
    if 'define' in header_line:
        md_dict['array_dimensions'] = {'x_dimension': ...}
    elif 'Content' in header_line:
        md_dict['data_type'] = header_line[header_line.index('Content') + 2]
    elif ...:
        ...
    else:
        # what do to about lines that match nothing?
        pass
```