Split vowel height into high, mid, low and lax.

The vowel height values can be defined as:

        hgh mid low lax
    ===================
    hgh  +   -   -   -
    smh  +   -   -   +
    umd  +   +   -   -
    mid  -   +   -   -
    lmd  -   +   +   -
    sml  -   -   +   +
    low  -   -   +   -

Where -lax is a tense vowel and +lax is a lax vowel.