ENH: Support incremental preprocessing

[corneliusroemer]

For the use case of processing large amounts of sequence data that are mostly changed only by addition of new sequences, it might be nice if preprocessing supported an incremental mode, that uses the previous processing result to speed up preprocessing.

I don't know the internals of what preprocessing does - but this could be very useful for use cases that want to do frequent updates with minimal delay.

[Taepper]

This is definitely on our TODO :)