Improve Tox configuration

[joaomcteixeira]

Improved tox env isolation:

• only py36 and py37 install runtime dependencies
• other envs act only on the src files and do not perform installation nor deps installation
• docs env now matches (hopefully) the ReaTheDocs build env, related with ReadTheDocs Mock configuration #16 mock configuration and does not use tox installed taurenmd dependencies.

During this process I faced a problem with coverage report results. With the tox config previous to this PR the coverage was reporting correctly, but with the separation of testenv from py36 coverage results diverge and decorator lines are registered as uncovered, but only in online repositories, locally coverage reports properly. I ended up removing the clean and report envs and add those as command_pre and command_post options, I found this to solve the solution locally.

I did try as much as I could solutions, may be in a future PR this can be solved. For know I am closing it, because improvements in tox env management really pay off.

Useful links for future reference:

• https://coverage.readthedocs.io/en/coverage-5.0.3/faq.html
• https://pytest-cov.readthedocs.io/en/latest/plugins.html
• Decorator Function marked as Missed pytest-dev/pytest-cov#301
• Document dealing with covering plugins pytest-dev/pytest-cov#177
• Incorrect coverage report pytest-dev/pytest-cov#117

I think the problem is that reflected in the first link, but I do not see why this configuration does not solve the problem, SPECIALLY, because the problem happens during the transfer from Travis-CI and coverage servers, locally and on travis-ci logs everything looks good. I am too puzzled.

This PR is a repeat from #18 because I though coverage reports there (in coverage servers) were getting contaminated from previous commits.

[codecov-io]

Codecov Report

Merging #19 into master will decrease coverage by 1.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #19      +/-   ##
==========================================
- Coverage     100%   98.96%   -1.04%     
==========================================
  Files          21       21              
  Lines         952      962      +10     
  Branches       81       81              
==========================================
  Hits          952      952              
- Misses          0       10      +10

Impacted Files	Coverage Δ
src/taurenmd/libs/libplot.py	77.77% <0%> (-22.23%)	⬇️
src/taurenmd/libs/libmda.py	86.95% <0%> (-13.05%)	⬇️
src/taurenmd/libs/libmdt.py	91.66% <0%> (-8.34%)	⬇️
src/taurenmd/libs/libcalc.py	98.27% <0%> (-1.73%)	⬇️
src/taurenmd/libs/libcli.py	98.98% <0%> (-1.02%)	⬇️

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[codeclimate]

Code Climate has analyzed commit 944f524 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (85% is the threshold).

This pull request will bring the total coverage in the repository to 98.9% (-1.0% change).

View more on Code Climate.