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.

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[shivammehta25]

Should we not add these configs to the main repo? This seems to be very dataset specific. Or do you think it is better to have it here?

[shivammehta25]

I think the one's that are publically available are fine, but perhaps mm-conv etc. are not publically available right now. So lets put the ones that are publically available. What do you think?

[jimregan]

I think for the moment, I'll cut out the en-us ones into their own pull request. The rest need extra language bits.