use qol methods in sources

1. self.num_samples and self.shape are still used in FakeSeriesSrc, so I defined them in TSSource. They can probably be reworked so they are not explicitly needed, but I'm not sure if that's worth the work.
2. I made the prepare_frame data initialization optional, and default to None. Seems like in most cases we are reassigning self.data to a new array, I am not sure what existing examples there are where initializing data to a zero array is better? One example that would use a zero array most of the time is the impulse source, but that can be done in the source element itself(?)