Add convolve for DiscreteNonParametric

[iampritishpatil]

DiscreteNonParametric convolution has a very nice trivial closed form. It was not implemented.

This pull request implements it.

[iampritishpatil]

Thanks for the suggestions, they were, all on the point.

I think I addressed all of it. If I run into any more floating-point tricky cases, I'll add them, but as far as I can tell, it shouldn't cause errors, apart from any unexpected compiler optimization which causes different things to happen in the two loops.

Let me know if anything more is needed.

[codecov-commenter]

Codecov Report

Merging #1523 (f85407a) into master (1c44a2a) will increase coverage by 0.02%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1523      +/-   ##
==========================================
+ Coverage   85.45%   85.47%   +0.02%     
==========================================
  Files         128      128              
  Lines        7818     7829      +11     
==========================================
+ Hits         6681     6692      +11     
  Misses       1137     1137              

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

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[mharradon]

I think this would be a nice PR to merge. I implemented the latest suggestions from @devmotion and PR'd at #1850. Happy to either see this PR fixed up or merge that.

Thank you!