Remove Gensim

[ajdapretnar]

Is your feature request related to a problem? Please describe.
Gensim apparently causes quite some issues on installation and in some widgets.
It is used is:

1. Corpus and Preprocessing: gensim.corpora.Dictionary
2. BOW: gensim.models.TfidfModel
3. in util: Sparse2CorpusSliceable
4. Topic Modeling: GensimWrapper

Describe the solution you'd like

1. Write own Dictionary (or copy from Gensim)
2. Use sklearn.feature_extraction.text.TfidfTransformer
3. Figure out Sparse2Corpus (minor)
4. Use LDA and other decomposition methods from sklearn.

Describe alternatives you've considered
SpaCy could also be used for TM and similar.

[ajdapretnar]

See also: #1069

[krystofair]

and: #1136