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