[script,engine,test] Allows the user to specify the number of steps for threshold/performance analysis

[utils.metric] Rename as measure as some of the values are not real metrics; Add tests for AUC and improve its implementation

[utils.plot] Implement a more fair std.dev. plot using a bivariate gaussian modelling instead of vector addition (thinner regions)

Streamlining

See merge request bob/bob.ip.binseg!12