[WIP] Effective exponent plots
Created by: rabah-khalek
Similar to normalize() in PDFPlotter, extern_function() computes the exponents.
PDFPlotter takes now additional arguments: function and its label (perhaps we should include them into one tuple later together also with the x range).
Labeling of the plot is not done yet, only alpha and beta effective plots.
two new functions added in eff_exponents.py: plot_alfaEff
and plot_betaEff