adds a new gene product engineering tool to encompass tools that allow to modify a gene product in a defined manner, and re-classify existing such tools accordingly;
adds a subclass of protein cleavage tag to refer to cleavage tags that are recognised and acted upon by a protease (to distinguish from cleavage tags that do no involve the action of a protease);
adds a term to represent proteases, as a gene product engineering tool.
This PR:
Closes #200