LdaMallet show_topic should use topn parameter name to match other topic models

[tmylk]

There is an inconsistency between show_topic-topn and show_topics-num_words in LdaModel which is replicated along other topic models in order to standardise the api.

show_topics and print_topics were standardised among topic models in #755 to match LdaModel to use num_words as the argument names were confusing.
Later #771 changed show_topic and print_topic to use num_words just in ldamallet. That was a way to standardise it with show_topics, but in retrospect it was not the best move.
Then print_topic was changed back to use topn like all the other models do when ldamallet inherited from BaseTopicModel. Though show_topic has remained - that is the inconsistency that should be resolved here.