Convert API so it complies with MLLib

[rjagerman]

The API for computing the LDA model and obtaining the result is different from the standard MLLib implementations (e.g. the online or EM LDA implementations). This can make it difficult to use in practice.

Changing our implementation to comply with the MLLib implementation would improve usability and integration into existing codebases. For example, the code can then be used in MLLib pipelines and corresponding evaluation routines.