Restore SSIM_KERNEL and SSIM_W

[Octogonapus]

This PR restores two globals, SSIM_KERNEL and SSIM_W, which were removed in v0.3.1.

Fixes #50.

[codecov]

Codecov Report

Merging #51 (6b61385) into master (83e91c5) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #51      +/-   ##
==========================================
- Coverage   96.75%   96.68%   -0.08%     
==========================================
  Files           7        7              
  Lines         185      181       -4     
==========================================
- Hits          179      175       -4     
  Misses          6        6              

Impacted Files	Coverage Δ
src/msssim.jl	100.00% <100.00%> (ø)
src/ssim.jl	100.00% <100.00%> (ø)

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[johnnychen94]

Sorry! I didn't expect people to use this internal constant so changed this without notification.

Can you also help revert the constructor of MSSIM? Thanks

[johnnychen94]

Thanks for the quick fix! I'll tag a release once the CI is green