Integration of asv benchmark

[agoscinski]

Draft of asv benchmark. Can be run with tox -e benchmarks

[agoscinski]

Float32 fail and CI runs through. Need to find a way that a failure in benchmarks results in a failure of CI

[agoscinski]

Because the computation of the input for the code that is benchmarked is very slow (30s on CI), we need to cache it on disk (class variables don't work, they are also multiple times initialized). Main problem for caching the inputs is that one needs to pickle them. The current Spline is not pickable, because it uses local definition of functions (equistore.Labels is also not pickable, but the workaround is rather simple). I would proceed when Luthaf/rascaline#212 is merged and then using the same Spliner to also not have duplicated code in the two repos. That introduces a dependency on rascaline, but so be it.