Wsknn

- (bug) changed behavior of the `WSKNN.recommend()` method: https://github.com/nokaut/wsknn/issues/67
- (feature) added functionality to transform input session-items and item-sessions dataframes to WSKNN input: two new functions (`dataframe_to_item_sessions_map` and `dataframe_to_session_items_map`))
- (docs) Added badges to README

- (bug) `random.sample(sessions)` bug: https://github.com/nokaut/wsknn/issues/64
- (enhancement) check if sessions and items have the same datatypes in both mappings (session-items map and item-sessions map) (https://github.com/nokaut/wsknn/issues/65)
- (enhancement) batch prediction with `batch_predict()` function

- (docs) updated JOSS badge,
- (docs) updated docs,
- (enhancement) updated import paths,
- (feature) save and load model with `joblib`

- (docs) changed README example,
- (docs) added `demo-readme` example to `demo-notebooks` section,
- (docs) demo example in documentation updated,
- (docs) link to the documentation page added to README,
- (enhancement) item-sessions map may be derived from session-items map,
- (feature) data can be read from the flat record structure,
- (feature) data can be parsed from dataframes

- support for `csv` files,
- debugged `gzip` loading.

- updated documentation,
- updated minimal Python version,
- added CI testing.