yaml parser: only parse relevant YAML data file for single-table conversion

[GraemeWatt]

Currently, the YAML parser reads all YAML data files into a tables list even if only one table is requested in the output. This is very inefficient. Moreover, the single-table conversion can fail for very large records like https://www.hepdata.net/record/ins2132368 (313 data tables) that cannot be parsed within the 5-minute server timeout limit. The YAML parser should check for the presence of a table option and only parse the relevant table into the tables list.