Add power mean functions

[delphidabbler]

Power mean

A power mean (or generalised mean) is a mean of the form

where all x are >= 0.

Source: Statistics How To & Wolfram MathWorld

function PowerMean(const A: array of Extended; Lambda: Extended): Extended;
begin
  Assert(not IsZero(Lambda));
  Assert(Length(A) > 0);
  var Sum: Extended := 0.0;
  for var X in A do
  begin
    Assert(Sign(X) <> NegativeValue);
    Sum := Sum + Power(X, Lambda);
  end;
  Result := Power(Sum / Length(A), 1 / Lambda);
end;

Weighted power mean

There's also the weighted power mean:

When all weights w sum to 1 then denominator is 1 and can be ignored.

Source: Statistics How To and Wikipedia

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This issue was extracted from issue #16

[delphidabbler]

Double, Integer and Cardinal overloads of PowerMean and WeightedPowerMean implemented by merge commit 59c09cc