ConcentricFiltration(K,n):: PureCubicalComplex, Int --> FilteredPureCubicalComplex

Inputs a pure cubical complex K and integer n \ge 1, and returns a filtered pure cubical complex of filtration length n. The t-th term of the filtration is the intersection of K with the ball of radius r_t centred on the centre of gravity of K, where 0=r_1 \le r_2 \le r_3 \le \cdots \le r_n are equally spaced rational numbers. The complex K is contained in the ball of radius r_n. (At present, this is implemented only for 2- and 3-dimensional complexes.)