0.1.0-alpha3

Incremental changes:

• Fix re-pluralisation of units not actually working in 0.1.0-alpha2.
• Configure development tools in pyproject.toml.
• Fixes to documentation.
• Fixes to NYT data.
• Additional sentence features:
  • is_stop_word
  • is_after_comma
• Only create features that are possible for the token e.g. there is no prev_word for the first token, so don't create the feature at all instead of using an empty string.
• Refactor code for easier maintenance and flake8 compliance .