Make taxcalc an optional dependency

[MaxGhenis]

Fixes #39 using the pandas approach specified there.

I've tested that taxcalc-dependent functions like calc_df correctly throw an error when taxcalc isn't installed, and work normally when taxcalc is installed.

[pep8speaks]

Hello @MaxGhenis! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2019-12-05 18:49:58 UTC

[codecov]

Codecov Report

Merging #52 into master will increase coverage by 4.19%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   66.66%   70.85%   +4.19%     
==========================================
  Files          22       24       +2     
  Lines         423      501      +78     
==========================================
+ Hits          282      355      +73     
- Misses        141      146       +5

Impacted Files	Coverage Δ
microdf/taxcalc.py	73.91% <100%> (+1.18%)	⬆️
microdf/_optional.py	100% <100%> (ø)
microdf/tests/test_taxcalc.py	75% <55.55%> (-25%)	⬇️
microdf/tests/test_optional_dependency.py	97.22% <97.22%> (ø)

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