Grzegorz

- fix `fetchipa`:
- don't hardcode the number of parallel processes
- fix `UnicodeEncodeError` when writing files

- `generate`: when optimising, keep `sj` and `zj` phoneme contrasts
- refactor comments

- `generate`:
- add `--no-phonemes`, `--keep-chronemes`, `--keep-stress` options
- remove `--ignore-stress` option
- refactor code structure

- fix: when converting a dictionary to a `Word`, add slashes around its IPA, so
that Anki flashcards look neater
- change aspect of the Anki flashcards

- fix: change required Python version to 3.10 so `match` is supported
- fix: use `with` statement when handling file I/O
- fix: try-except on specific errors, not any
- README:
- add 'From source' installation instructions
- expand the IPA abbreviation to avoid ambiguity

- fix phoneme, chroneme and stress contrast checkers (ALL had bugs!)
- fetchipa: don't store words that have no IPA spelling