TSSS missing a factor of 2

[jlmelville]

This was reported to me at jlmelville/rnndescent#8 but would also affect pynndescent, which is unsurprising as I directly converted the equation from here into C++.

Equation 7 of the TS-SS equation in the original paper (PDF), written in terms of degrees, has a denominator of 720, which would convert to 4pi in radians (the pi would cancel). As can be seen at:

pynndescent/pynndescent/distances.py

Lines 518 to 520 in d99a821

	sector = ((np.sqrt(d_euc_squared) + magnitude_difference) ** 2) * theta
	triangle = norm_x * norm_y * np.sin(theta) / 2.0
	return triangle * sector

there's only a division of 2 that shows up in the calculation of the return value.

I would volunteer to write a PR and update any affected test, but I would like to to use that paper as a means of confirming my understanding of the equations, as there are some partial numerical results in it. Unfortunately, I quickly got fed up trying to back-convert the vectors in the figures to plausible 2D descriptors (e.g. how can A and B in figure 1a be vectors of length 6, with an angle of 30 degrees between them, but a Euclidean distance of 2). Obviously this doesn't affect the returned neighbors but it could have a (small) effect for anything that used the distances downstream, like UMAP.