Accuracy and parallel computation in parton density calculation

This project is about adding optimised and accurate high-order interpolators to the LHAPDF C++ library. This is required for adding support for Chebyshev polynomials, which are a very efficient way to parametrise Parton Density functions. Parametrising PDFs is essential to studying the high energy collision experiments that happen between protons in the LHC, as they encode details on the internal structure of a proton.