Add benchmarks

[Kolaru]

This PR add benchmarks using PkgBenchmark.jl.

These benchmarks are based on the Smiley and Chun examples used in tests and the Dietmar and Ratz example functions used in the /perf folder. I only added a subset of the benchmark performed in the /perf folder to keep the benchmark time reasonnable, while having representative benchmarks.

Of course, I will gladly add anything that seems missing.

This was tested with PkgBenchmark.jl v0.2.0.

No special infrastructure are added by this PR to automatically benchmark the package, as I don't know if it is worth for now (or will ever be).

I see this PR as usefull to the progress of #97, as it will facilitate discussion about its performance consequences.

[codecov-io]

Codecov Report

Merging #105 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #105   +/-   ##
=======================================
  Coverage   58.48%   58.48%           
=======================================
  Files          11       11           
  Lines         542      542           
=======================================
  Hits          317      317           
  Misses        225      225

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[Kolaru]

Codecov gets very confused but I don't know why. Putting back tests in their initial form does not help.

[Kolaru]

Build is failing because some tests in newton1d.jl fail due to the latest master of IntervalArithmetic.jl. They are fixed in #93 though, if needed I could submit a separate hotfix though.

[Kolaru]

Unless some more benchmark are requested I thing this is ready.

[dpsanders]

Thanks a lot!