Project tip frequencies once and slice the resulting array as needed
Project tip frequencies once and slice the resulting array as needed

Projection of frequencies happens at the tip level, so tip frequencies
can be projected once and the resulting array will be future tip frequencies
in the same order as the original array of frequencies, `freq_arrays`.

nextstrain/augur,blab/nextstrain-augur,nextstrain/augur,nextstrain/augur