Add vecjac operators

[avik-pal]

No description provided.

[ChrisRackauckas]

We have Zygote under @requires right now for https://github.com/JuliaDiff/SparseDiffTools.jl/blob/master/src/differentiation/jaches_products_zygote.jl . Looking at the dependency list on JuliaHub and the stability of Zygote, I think we might want to keep it under Requires.jl for now.

[codecov-commenter]

Codecov Report

Merging #158 (7974f57) into master (867ccda) will decrease coverage by 3.61%.
The diff coverage is 70.83%.

@@            Coverage Diff             @@
##           master     #158      +/-   ##
==========================================
- Coverage   83.72%   80.11%   -3.62%     
==========================================
  Files          12       13       +1     
  Lines         688      724      +36     
==========================================
+ Hits          576      580       +4     
- Misses        112      144      +32     

Impacted Files	Coverage Δ
src/SparseDiffTools.jl	100.00% <ø> (ø)
src/differentiation/vecjac_products_zygote.jl	0.00% <0.00%> (ø)
src/differentiation/jaches_products.jl	97.87% <96.59%> (+0.06%)	⬆️

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