Refactor HisparcMessageFactory and add tests

[davidfokkema]

Lots of duplication and complex logic. I think we should create a generic MessageFactory which accepts a class and a message, and uses simpler logic. Then, create two factory functions: HisparcMessageFactory and GpsMessageFactory. They simply call the generic factory with their respective classes.

As for the complex logic: we can drop a while loop if we let strip_until_start_codon raise an exception if there is no start codon left in the buffer. And we can probably drop the recursive call, although that may be not be a problem. Anyway: simpler logic, and test the thing!