In var, don't compute the mean if provided

[knuesel]

As mentioned in #12, var currently calculates the mean even if provided through the mean keyword argument.

Before this PR:

v = rand(100000);
μ = mean(v);

julia> @btime var($v, mean=$μ)
  33.777 μs (0 allocations: 0 bytes)

After:

julia> @btime var($v, mean=$μ)
  17.571 μs (0 allocations: 0 bytes)

[codecov]

Codecov Report

Merging #68 (b710298) into master (ba90d86) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
+ Coverage   97.37%   97.41%   +0.04%     
==========================================
  Files           1        1              
  Lines         381      387       +6     
==========================================
+ Hits          371      377       +6     
  Misses         10       10              

Impacted Files	Coverage Δ
src/Statistics.jl	97.41% <100.00%> (+0.04%)	⬆️

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[nalimilan]

Thanks!

[DilumAluthge]

@nalimilan Good to merge now?