Added seed phrases to KeyNMF

[x-tabdeveloping]

You can now add a seed_phrase to a KeyNMF model, which essentially indicates the aspect, from which the model has to examine documents.

from sklearn.datasets import fetch_20newsgroups

from turftopic import KeyNMF

corpus = fetch_20newsgroups(
    subset="all",
    remove=("headers", "footers", "quotes"),
).data

model = KeyNMF(5, seed_phrase="Is homosexuality moral?")
model.fit(corpus)

model.print_topics()

Topic ID	Highest Ranking
0	homosexuality, homosexual, immoral, sodom, heterosexual, sexual, fornication, christians, verses, sex
1	morality, moral, immoral, morals, objective, morally, society, animals, behavior, natural
2	christians, christian, christianity, religion, bible, god, church, religious, faith, beliefs
3	homosexual, homosexuals, heterosexual, gay, sexual, sex, heterosexuals, straight, men, sexuality
4	sin, sins, god, sinner, sinful, condemnation, sinned, scripture, punishment, sinners

TODO:

• Add documentation
• Release new version

[x-tabdeveloping]

@KennethEnevoldsen can I has review?

[KennethEnevoldsen]

Looking great a few ideas to restructure the docs - nothing holding back this PR though