Add an option to set-up the length cutoff between the approximative and exact
modes

Add a copyright windows to the perlTk interface, where to print the GPL

Put all the messages as macro in order to ease correction and, why not,
translations.

Merge mismaches NN and regular NN in one file. Treat every hybridations the same
way, just with different parameters.

Detect automatically the self-complementarity, and change the correction
accordingly.
