Optimise d3.mean.

Originally we were using Welford’s algorithm, but this is primarily
useful when computing the variance in a numerically stable manner, since
Welford’s approach requires an incremental mean.

I’ve removed a test for the mean of more than one instance of
Number.MAX_VALUE as this is unlikely to occur in practice; most likely
this was the reason I used Welford’s algorithm in the first place.

There’s a paper [1] comparing various algorithms for computing the mean,
and Welford’s is actually slightly less accurate than the naïve
approach.  There are some more accurate approaches but I think it’s
overkill for d3.mean.

[1] Youngs, Edward A., and Elliot M. Cramer. "Some results relevant to
choice of sum and sum-of-product algorithms." Technometrics 13.3 (1971):
657-665.

Related: #1842.