Add ability to specify TR/MH chain when standardising

[yutanagano]

When applying tr.standardize, one may already know that the gene should be an alpha or beta chain (even if the input string/symbol does not make this clear). Sometimes, this may add crucial information that is the difference between a standardisation success/failure. It would be useful to add the option for the user to specify whether it is an alpha or beta chain they are standardising. (feedback courtesy of @mm523)