Document LDA-related models (#2026)

* Started documenting the LDA module. This is WIP, squash this commit later

* More work on the LDA module

* Started documenting the callbacks module

* Finished documenting the Callbacks module

* Finished documenting LdaModel, example fix still pending

* Added documentation to the multicore version of LDA. Examples still need to be fixed.

* applied code review corrections

* temporary chunk of work on the sequential lda model

* Fixed examples

* Fixed examples for multicore LDA

* Fixed examples in the callbacks module. EpochSaver raises a pickling error when binded to the model so I removed it

* trivial fix

* Added documentation for the DTM. Inspection and feedback probably needed.

* Applied code review corrections

* Documented part of the SSLM helper class.

* Finished documenting all helper classes in the sequential LDA module

* Improved module level docstring example

* Fixed example

* fix pep8

* formatting fixes in LDA based modules

* fix formatting in ldaseqmodel (module seems broken)

* update lsimodel docs

* revert breaking code-changes

* fix PEP8 and documentation build

* cleanup callbacks

* cleanup ldaseqmodel

* cleanup ldamodel

* cleanup ldamulticore (+ add link to onlineldavb.py script to ldamodel.py)