optimize_epoch: run one epoch of the umap optimization
In donelsonsmith/umap_R: Uniform Manifold Approximation and Projection

run one epoch of the umap optimization

1	optimize_epoch(embedding, pairs, adjust, nns, abg, alpha)

`embedding`	matrix with embedding coordinates (coordinates along columns)
`pairs`	matrix with rows: from to; indexes must be 0-based
`adjust`	vector with 0/1 whether to adjust or not
`nns`	vector with negative-neighbors-set size
`abg`	vector with configuration parameters, a, b, gamma, move_other
`alpha`	numeric learning rate for this epoch
`book`	matrix with columns: eps, epns, eon2s, eons, nns (in that order)