LDA Incremental Updates

[justjhong]

WIth the addition of AmortizedLDA, we would like to make incremental updates to improve the model. This issue will serve as the parent issue for a bunch of small features we would like add to the LDA model.

• [x] Tutorial for LDA #1195
• [x] User Guide for Amortized LDA #1178
• [ ] Automated selection of n_topics
• [ ] Non-amortized LDA model
• [ ] prodLDA flavor of LDA
• [ ] correlated topic model
• [ ] Integrate with enrichR or expose an API for enrichment analysis of LDA topics

[adamgayoso]

Also if the normal prior has a full covariance matrix it becomes the correlated topic model...