REF: use new pytables helper methods to de-duplicate _convert_index
The assertions in here are intended to be temporary.  Once reviewers are convinced this is accurate, then the assertions can be removed and a bunch of these blocks can be condensed down to share code.

The new helper func _get_data_and_dtype_name is also going to be used later to pre-empt the need for the state-altering `get_data` method.