Add registry.get_formats() function to return table of formats
Check for no formats matching input data_class
Auto-update unified I/O class read and write docstrings to show available formats
Include available formats in exception message for read/write fail

Some minor comment additions and docstring update as well.
Fix minor doc mistake
Add deprecation of short-form ascii format names
Add formats table to unified I/O docs
Make doc update work without pre-existing formats table
Fixes so that test_registry tests pass
Fix Python 3 failure when updating docstring
Rename variable has_read to has_readwrite for clarity
Sort formats table by Deprecated column to put them at the end
Print available formats table when format could not be identified
Fix io.registry tests
Merge pull request #1669 from taldcroft/io/get-formats

Add function to get unified I/O formats and auto-update read/write docstrings