The newer version of cohere can return incorrect types.

Need to specify texts is the first arg for cohere v^5.00.

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.

Ensured that splitters embed in the __call__ method, rather than on initialization.

Also updated the Conversation class to uttilize splitters in assigning topic ids to Messages as the list of Messages grows.