This could be made into a class method (```@classmethod```) so that it can be called from multiple contexts (i.e., so that users aren't forced to first instantiate a ```BiologicalSequence``` to check if a character is a gap character or not).

I'm thinking something like:

```
@classmethod
def isGap(cls, char):
    return char in cls._gap_alphabet
```

This should also work for subclasses of ```BiologicalSequence``` without having to reimplement.