Add presumed PDF tables

[michael-a-hansen]

Adding the capability to build tables 'appended' with mixing variables as in presumed PDF models would enable direct application of data from Spitfire in solving turbulent combustion problems with RANS/LES. This is currently a major gap that could slow adoption of Spitfire in 'real' simulation codes. Recent work developed this capability using clipped Gaussian PDF code from the TabProps code at the University of Utah and SciPy integration, together with built-in multiprocessing, to accomplish efficient, accurate PDF convolutions. This work could be expanded, tested, and incorporated into Spitfire reasonably quickly. A nice feature of using Python to drive the convolution integrals would be user-defined PDFs.