DEV: New branch including only avizo file IO related files

Removed fileops.py so that pull requests focused on avizo file-IO
will only include updates to the avizo file-IO functions. Fileops
is also being reduced to a series of files each focused on a separate
file type IO functionality.
DEV: Pythonic improvements to code as per Tom's comments
DEV: Changed try/except to appropriate if/elif code commands

While the try/except format that was included originally worked,
using try/except without expecting an exception to be raised
when the try function fails is an inappropriate use of the try
function. Using if/elif statements is more appropriate for what
I am trying to do, which is to selectively populate a metadata
dictionary by reading lines from the original header string.
Merge branch 'avizo_io' into file_io