Changes to the code meant we needed to change some pytests.

New pytests added for Conversation splitter method.

Also fixed a bug in cumulative_sim.py where we weren't limiting curr_split_docs to only latter docs after a split.

We need to figure out how to check if first two messages belong to the same topic before implementing.

At the moment they code is written such that the first two messages automatically are sorted into the same topic.

The splitting code needed tidying and docstrings.

Delegated tasks in split_by_topic() to newly created methods to reduce number of lines of code in split_by_topic().

The split_by_topic was adding the last message from the previous splitting to the current splitting output, resulting in duplicates.

I also made the code more readable.

Also moved Conversation from schema.py to text.py to avoid cyclic imports.