Added new merging type based off pandas merge as of. Lets you merge the left time column with the most recent record in the right time column. For more details read: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.merge_asof.html
Also added filter to select for highest numbered version of a given record. Data is grouped by the remaining primary key columns and the record with highest version number is kept. Should only be used on single AEMO tables that have not been merged with other AEMO tables.