Expose cython API for external use

[kylebarron]

Change list

• Update CMakeLists.txt to include .pyx and .pxd files in distribution after skbuild is done processing
• Include h3api.h in final bundle. This can usually be found automatically when compiling, but if we have issues with this in the future, we can emulate Numpy and its np.get_include function.
• Add test that uses the external Cython API.

Closes #152

[codecov]

Codecov Report

Merging #234 (d951b5e) into master (3d09da1) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #234   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           17        17           
  Lines          425       425           
=========================================
  Hits           425       425           

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[kylebarron]

Ah the test script in cibuildwheel needs to be updated to either ignore the Cython test or compile the cython file used in that test.

[kylebarron]

I updated to ignore the test when the cython example wasn't compiled

[ajfriend]

Sweeeeeeet!

[ajfriend]

Really simple notebook example for future reference: https://gist.github.com/ajfriend/167afffde74bc08b0e809e10b92ecde3