Dataset download/conversion

[jimregan]

What does this PR do?

This adds scripts to download and convert LJspeech, hificaptain, and mcspeech.

The download locations for LJspeech and hificaptain were made to match what was in the existing yaml files, though hificaptain needs some tweaking.

Before submitting

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• [ ] Did you make sure your PR does only one thing, instead of bundling different changes together?
• [ ] Did you list all the breaking changes introduced by this pull request?
• [ ] Did you test your PR locally with pytest command?
• [ ] Did you run pre-commit hooks with pre-commit run -a command?

Did you have fun?

Make sure you had fun coding 🙃