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.

conda-recipe/meta.yaml

@@ -52,6 +52,7 @@ requirements:
         - sphinxcontrib-bibtex
         - docutils =0.16 # This dependency is not explicity needed but https://github.com/NNPDF/nnpdf/issues/1220
         - curio >=1.0
+	- findiff
 
 test:
     requires: