Support complex pullbacks

[sethaxen]

Replaces _dot with _realdot, which allows for pulling back through functions with complex outputs. Resolves #22. This is currently untested.

[codecov-commenter]

Codecov Report

Merging #23 (9bab121) into master (b4343f5) will decrease coverage by 1.15%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master      #23      +/-   ##
==========================================
- Coverage   78.26%   77.10%   -1.16%     
==========================================
  Files           1        1              
  Lines         345      345              
==========================================
- Hits          270      266       -4     
- Misses         75       79       +4     

Impacted Files	Coverage Δ
src/AbstractDifferentiation.jl	77.10% <16.66%> (-1.16%)	⬇️

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[mohdibntarek]

Could you please add a test here for the intended use case?

[sethaxen]

Certainly, though the tests need a few refactors first to make testing these cases more straightforward. Some changes made in #27. More will be necessary.

[sethaxen]

Also, this PR should use RealDot.jl now that it exists.