Add walsh(n) function that returns a Walsh matrix

[syoshida1983]

I added a function walsh(n) that returns a Walsh matrix by bit-reversal permutation of a Hadamard matrix. The following package provides a similar function but merges it into Hadamard.jl would be helpful.
https://github.com/syoshida1983/Walsh.jl

[stevengj]

Also needs some tests, e.g.

@testset "walsh(n)" begin
    @test walsh(8) == [1 1 1 1 1 1 1 1; 1 1 1 1 -1 -1 -1 -1; 1 1 -1 -1 -1 -1 1 1; 1 1 -1 -1 1 1 -1 -1; 1 -1 -1 1 1 -1 -1 1; 1 -1 -1 1 -1 1 1 -1; 1 -1 1 -1 -1 1 -1 1; 1 -1 1 -1 1 -1 1 -1]
    @test_throws ArgumentError walsh(24)
    for n in 2 .^ (0:10)
        @test fwht(Matrix(I(n)), 1) * n ≈ walsh(n) rtol=1e-13
    end
end

[codecov]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.84%. Comparing base (047c8fc) to head (ddbb690).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Hadamard.jl	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   78.23%   78.84%   +0.61%     
==========================================
  Files           1        1              
  Lines         147      156       +9     
==========================================
+ Hits          115      123       +8     
- Misses         32       33       +1     

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[syoshida1983]

Thank you for your review. I have incorporated the suggested changes and tests into the code and verified that the tests passed.

[stevengj]

You should also add a note about the walsh(n) function to the README, in the section at the end about Hadamard matrices.

[syoshida1983]

I updated README.md.