Fix: move sequence truncation into BatchConverter

[Alexperiments]

Fixes Issue #157

• I added the truncation logic in the __call__ of BatchConverter and added a truncate parameter in its constructor.
• I checked that BatchConverter is accessed through the get_batch_converter method of Alphabet, therefore I added a truncate parameter to this method.
• I accordingly modified all the calls to this method in the rest of the code.
• In the test scripts, I set truncate=True since it is the default behavior.

As a side note: I've just noticed that the commit I made to sync my fork is listed in my PR. This is not intended, but I don't know how to remove it directly. If the PR gets approved I can eventually drop it while rebasing.

[Alexperiments]

Thank you @tomsercu for your feedback. Tell me if there is something else that I need to change.