Tub improvements:

* Fixed manifest iterator to avoid too deep recursive function calls which segfaults when deleting many records
* Automatic addition of '_session_id' field in the Tub. This mimics the naming convention we used to have for old-style tubs before: YY-MM-DD_N, where N is a running integer. All session ids of a tub are stored in the manifest.json file. The tub needs to receive records and needs to get closed for this entry to be updated.
* Added test for the new tub and tub writer logic

Tub GUI development:
* Support starting from 'donkey ui' command
* All relevant settings are stored in ~/.donkeyrc file which is editable

Training improvements:
* You can now put a predicate for tub records into the myconfig.py to filter records for training. The config variable TRAIN_FILTER has to be set to that function.
* Improve training performance by using tf prefetch
* Default AI framework to 'tensorflow' in training

Others:
* Update docs
* Update patch version number to 4.2.0
* Reduced run time of test_train using only 5 epochs and adjusting thresholds accordingly
* Reduced run time of torch tests using 3 epochs - these were very slow (like 25+ m on MacBook)
* Minor updates in docstrings and switching to f-strings
* Fixed duplicated function in keras.py