A complete rewrite of generate_site_positions.py

This is a complete rewrite of generate_site_positions.py.
The new version runs under python3 as well as python2.
It corrects an error found in the previous version where the very last number in the result was wrong (it was one too many).
It can work with multiple test strings at the same time (they must be enumerated in a python list).
Wildcard search (using N, M, R, W, Y, S, K, H, B, V, D) is also possible with the new algorithm.
Compared to the previous version this program runs faster for simple test strings (strings without wildcards).
The script contains a dictionary for genome/filename mappings and another one for enzyme/test string mappings which users can modify easily according to their needs.