Adding Quadratic elements to Silo abstract knowledge pool

[cessenat]

This adds the quadratic elements ids for Silo. Next step shall be to make them properly readable in VisIt Silo plugin. Further step shall be to take a decision about extfaces.c : degenerate quadratic elements down to linear on boundaries so that very few work shall need to be done, or preserve the quadratic feature. I would opt for the first option, what do you think @markcmiller86 ? Another step shall be for mixed materials ; I think mixed materials is a typical Finite Volume issue, so high order Finite Elements are orthogonal to that question ; so, as for external faces, one could degenerate quadratic to linear for mixed materials.