Refit/stream for prophet

[JSzitas]

Hi!

I noticed that there is no refit method available for this. Would you be interested in a PR that would provide this?

[nipnipj]

What's the alternative to refit()?

[mitchelloharawild]

I would be happy to accept PRs for refit/stream, I haven't added it since the refitting/streaming capabilities were/are not available in {prophet}. Any pointers to the documentation / source which would allow refitting while keeping the parameters the same, or streaming by extending an existing model with new data would be useful.