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.