restrictionenzyme selector

This commit adds the functionality to select any of the restriction enzymes described in the rebase database (http://rebase.neb.com/rebase/link_emboss_e).
It also finds the position where the enzyme cuts, even though it does not consider the string circular. This is a major bug and will be fixed.

As of now there is no code for drawing the restriction sites into the plasmid view or any other GUI element.

This commit is just a first step.