Install heavy dependencies (e.g. transformers, torch) on demand

[hlibbabii]

Installation of Bohr takes a lot of time. This especially affects CI since it is run often. The solution is to install some dependencies on demand.

This could be done with the requires_extra functionality of setuptools. For example:

• Among other dependencies, pip install 'bohr[classifiers]' would install transformers library, which is not installed by default;
• pip install 'bohr[test]' would only be needed for development purposes to install dependencies to run tests.

[hlibbabii]

Related to #155, but there it's about external tools run by heuristics. This issue, however, is about python libs used by the framework