Switch to building cython extensions at installation time

[djhoese]

We currently provide the C/C++ rendered files from Cython in our sdist and in the repository. This causes issues when Python changes its API and Cython needs to re-render the files to match. This PR changes the necessary pieces of the setup.py and CI so this rendering always happens on installation. Users without compilers will need to depend on the wheels on PyPI.

• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff
• Fully documented

[codecov]

Codecov Report

Merging #336 (606bfed) into master (49a3d7d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #336   +/-   ##
=======================================
  Coverage   92.59%   92.59%           
=======================================
  Files          48       48           
  Lines        9847     9847           
=======================================
  Hits         9118     9118           
  Misses        729      729           

Flag	Coverage Δ
unittests	92.59% <ø> (ø)

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[coveralls]

Coverage remained the same at 92.587% when pulling 1527a05 on djhoese:build-require-cython into 49a3d7d on pytroll:master.

[pnuu]

LGTM. This will clean-up also the diffs a lot for the following PRs touching the Cython code.

[mraspaud]

LGTM, thanks!