Accept arbitrary arrays also for var and std.

[GunnarFarneback]

Most functions were extended from Vector to AbstractArray in #20, but not var and std. I don't see a need for this inconsistency, so this PR loosens their type constraints. It also revises their docstrings to make a bit more sense.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.15%. Comparing base (b1040f3) to head (b0fbe83).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master      #98   +/-   ##
=======================================
  Coverage   94.15%   94.15%           
=======================================
  Files           1        1           
  Lines         154      154           
=======================================
  Hits          145      145           
  Misses          9        9           

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[devmotion]

Suggested change

	`NaNMath.var(A)`
	NaNMath.var(A::AbstractArray{<:AbstractFloat})

[GunnarFarneback]

I had trouble getting the normal four space indentation to mark it correctly but now I see that it's the 1.12 non-public warning notice that interferes.

[GunnarFarneback]

I agree that there are all sorts of improvements that could be made to this package but my ambitions don't go further than what's in this PR. Feel free to take it over and make a better PR if you want.