improved the toc function to return the time as well as print it.  added a 'quiet' argument as well to avoid printing.
Refinements to Tortuosity
- It returns the ratio of network distance to crow-flies distance between every two pores
- It now has a 'estimate_time' method that works quite well
OP now returns 'occupancy' as a float
- this accommodates late pore filling
- also plays nice with labels vs props methods