`mean` Was Added To `std.algorithm`

$(REF mean, std, algorithm, iteration) accurately finds the
mean (a.k.a the average) of any range of number-like elements.

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import std.algorithm.iteration : mean;
import std.math : approxEqual;

int[] arr1 = [1, 2, 3];
real[] arr2 = [1.5, 2.5, 12.5];

assert(arr1.mean.approxEqual(2));
assert(arr2.mean.approxEqual(5.5));

// user defined number types also work
import std.bigint : BigInt;

auto bigint_arr = [
    BigInt("1_000_000_000_000_000_000"),
    BigInt("2_000_000_000_000_000_000"),
    BigInt("3_000_000_000_000_000_000"),
    BigInt("6_000_000_000_000_000_000")
];
auto seed = BigInt(0);
assert(bigint_arr.mean(seed) == BigInt("3_000_000_000_000_000_000"));
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