changed function to return usefull list regarding restriction enzymes

As discussed I have added a variable called  genbank.restriction_sites to store the restriction information. It contains infos about the restriction sites in the following manner:
```
[[name, start, stop, cut 1, cut 2, recognised sequence],[...]]
example:
[['BamHI', 6090, 6096, 6091, None, 'ggatcc']]
```