Implemented function to compute asymptotic exponents

This PR implements functions to compute the asymptotic exponents in the form of plots and of a table of their expectation values and uncertainties at given x and Q. The definition of the asymptotic exponents is as in Eq. (4) of [arXiv:1604.00024]. Examples are provided; the functions are documented. New PDF bases have been defined to introduce PDF combinations that may be relevant for the computation of the asymptotic exponents.

validphys2/examples/asy_exponents/asy_exponents_plots.yaml

+meta:
+    author: Emanuele R. Nocera
+    title: Asymptotic Exponents
+    keywords: [asy_exp]
+
+pdfs:
+  - {id: "NNPDF40_nnlo_as_01180", label: "NNPDF4.0"}
+
+Q: 5000 # GeV
+
+alpha:
+  xmin: 1e-5
+  xmax: 1e-3
+  ybottom: 0.0
+  ytop: 1.0
+  basis: flavour
+  npoints: 70
+
+beta:
+  xmin: 0.6
+  xmax: 0.9
+  ybottom: 0
+  ytop: 10
+  basis: flavour
+  npoints: 70
+
+show_mc_errors: False
+
+template_text: |
+  ---
+  title: Asymptotic exponents
+  keywords: [asy_exp]
+  ---
+  Asymptotic exponents plots
+  -----------
+  {@alpha plot_alpha_asy@}
+  {@beta plot_beta_asy@}
+  -----------
+
+actions_:
+  - report(main=true)
+  
\ No newline at end of file