Add max by and min by aggregates

[texodus]

This PR adds two new aggregate types, min by and max by, as well as UI support and tests. These aggregates take two columns rather than one, similar to the weighted mean aggregate, and will return the value from the first column, which corresponds with the max row from the second. For this dataset:

const data = {
    a: [1, 2, 3, 4, 5, 6, 7, 8],
    b: [0, 1, 0, 1, 0, 1, 0, 1],
    c: ["h", "g", "f", "e", "d", "c", "b", "a"],
};

const config = {
    columns: ["c"],
    aggregates: {
        c: ["max by", "a"],
    },
    group_by: ["b"],
};

we can use the max by aggregate to select string from column "c" which corresponds to the maximum "a" for every group "b".