new english_mfa model

[veltzerdoron]

This models' IPA transcription symbols seem to have changed quite a lot from those used by the previous english_ipa, is that intentional?

Also... how and why this was done?

[noetits]

Some info on this are given here and here

[mmcauliffe]

Thanks @noetits! The updated link for the MFA phone set is https://mfa-models.readthedocs.io/en/latest/mfa_phone_set.html, the one you have linked is the original "refactor" branch that I've just turned off since it's very out of date now.