Add COO.imag, COO.real, COO.conj()

[jcrist]

Add methods and properties to support complext sparse arrays, matching the numpy api.

[codecov]

Codecov Report

Merging #196 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #196      +/-   ##
==========================================
+ Coverage   97.64%   97.65%   +<.01%     
==========================================
  Files          11       11              
  Lines        1441     1447       +6     
==========================================
+ Hits         1407     1413       +6     
  Misses         34       34

Impacted Files	Coverage Δ
sparse/coo/core.py	97.14% <100%> (+0.04%)	⬆️

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[hameerabbasi]

I'm not entirely sure if a Returns section would be appropriate in the docstring... Other than that, these comments are all I have.

Feel free to merge after these and a changelog entry.

[jcrist]

I believe I've resolved all issues. Thanks for the review, merging.