PoertoflioDataSets.ranking(): New postprocessing method
New plotting method rank_by_budget. Idea by Petr Posik.
plot_conv: Add support for 'envelope' reference strategy, i.e. bestalg

bestalg() is left envelope of all function evaluations.
PortfolioDataSets.ranking(): Rank converged algorithms by order of convergence
plot: Add support for rank_by_budget plot averaged over many functions
pplot rank_by_budget(): Fix x axis log base
plot_conv overview(0): -ert2oracle_by_ert, +rank_by_budget