speed up domain loading

[raoulvm]

Domain loading is extremely slow for large bots - about 5 Mins for a 3.3 MBytes domain (including responses), and that is due to multiple times loading and parsing the same file. This commit addresses that, be removing the validation if loading from a model tar.gz and loading the file(s) only once if reading from a directory.

Proposed changes:

• do not read the domain files multiple times, do the validation only once (when training), or skip it (when loading model)

Status (please check what you already did):

• added some tests for the functionality
• updated the documentation
• updated the changelog (please check changelog for instructions)
• reformat files using black (please check Readme for instructions)