[SPARK-9481] Add logLikelihood to LocalLDAModel

jkbradley Exposes `bound` (variational log likelihood bound) through public API as `logLikelihood`. Also adds unit tests, some DRYing of `LDASuite`, and includes unit tests mentioned in #7760

Author: Feynman Liang <fliang@databricks.com>

Closes #7801 from feynmanliang/SPARK-9481-logLikelihood and squashes the following commits:

6d1b2c9 [Feynman Liang] Negate perplexity definition
5f62b20 [Feynman Liang] Add logLikelihood