background never considered

[ianhi]

must document that the background cells are never filled in.

Add an option for doing this. or fill them in at the end?

[ianhi]

fast-overlap/fast_overlap/fast_overlap.pyx

Lines 61 to 62 in 89736ae

	if p and c:
	arr[p, c] += 1

and

fast-overlap/fast_overlap/parallel_overlap.cpp

Line 13 in 89736ae

if (prev_id && curr_id) {