matchms similarity: Add Spec2Vec module

[hechth]

Using Spec2Vec as a similarity metric by accepting a trained model as an additional parameter would be a cool feature!

For the procedure see here.

Relevant papers:
http://dx.doi.org/10.1371/journal.pcbi.1008724

Steps:

• Train the model on provided testing data
• Include in matchms similarity wrapper
• add wrapper for Spec2Vec model training #315
• add private wrapper to retrain Spec2Vec model from pickle binary #316

Problems:

• matchms: Custom GENSIM exporter for Spec2Vec trained model since pickle is not very safe #290

[maximskorik]

Adding a diagram from #290 to have a perspective of the proposed Spec2Vec workflow.