isShelling -- determines whether a list of faces is a shelling

Determines if a list of faces is a shelling order of the simplicial complex generated by the list.

Let $S$ be the simplicial complex generated by the list of facets $L$. If $S$ is pure, then definition III.2.1 in [St] is used. That is, $L_1, .., L_n$ is a shelling order of $S$ if the difference in the $j$-th and $j-1$-th subcomplex has a unique minimal face, for $2 \leq j \leq n$.

If $S$ is non-pure, then definition 2.1 in [BW-1] is used. That is, $L_1, .., L_n$ is a shelling order if the intersection of the faces of the first $j-1$ facets with the faces of the $L_j$ is pure and $dim L_j - 1$-dimensional.