Add arithmetic mean functions

[delphidabbler]

Delphi's Math unit has a Mean function, but only for Float arrays:

function Mean(const Data: array of Single): Single;
function Mean(const Data: array of Double): Double;
function Mean(const Data: array of Extended): Extended;

So we could define Mean for integers that returns a Double. Two possible implementations:

function MeanInt(const A: array of Integer): Double;
begin
  Result := 0.0;
  for var I := Low(A) to High(A) do
    Result := Result + A[I] / Length(A);
end;

function MeanInt2(const A: array of Integer): Double;
begin
  var Sum: Int64 := 0;
  for var Elem in A do
    Inc(Sum, Elem);
  Result := Sum / Length(A);
end;

The former is less likely to overflow and generalises to 64 bit integes better, while the latter is faster.

This issue was extracted from issue #16

[delphidabbler]

We could reimplement the Delphi means if they use the overflow prone implementation.

Our mean function could be named ArithmeticMean or ArithMean

[delphidabbler]

We could reimplement the Delphi means if they use the overflow prone implementation.

--- they are!

Our mean function could be named ArithmeticMean or ArithMean

--- going with ArithMean

Decided to implement 3 overloads: Integer, Cardinal and Double.

[delphidabbler]

Implemented Cardinal, Integer and Double overloads of the ArithMean function noted in #30 (comment).

Also added the same overloads of new WeightedArithMean functions that calculate the weighted arithmetic mean.

Implemented by merge commit a6d67b3