Fix asv benchmarks

[luizirber]

At some point we configured asv to run benchmarks, but it was never integrated into CI (or had many benchmarks implemented). Working on improving the situation =]

This is a start, making asv run without dependencies on tests/. It would be better to have a fixed set of data in every run (even thought is unlikely that the random data will be extremely biased for one specific comparison in the benchmarks), but maybe do that in #128.

I also added the first github action to the repo. This way we don't extend our running time on Travis. The benchmark check is not required (for now), but it is nice to have some performance tracking that doesn't required running asv locally or depending on PR submitter self-reporting.

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov-io]

Codecov Report

Merging #509 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #509   +/-   ##
=======================================
  Coverage   88.59%   88.59%           
=======================================
  Files          30       30           
  Lines        4727     4727           
  Branches       46       46           
=======================================
  Hits         4188     4188           
  Misses        536      536           
  Partials        3        3

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[luizirber]

Ready for review and merge @ctb @olgabot

[ctb]

LGTM