divorce b.N from number of points in chunk
divorce b.N from number of points in chunk

for 2 reasons:
- we want precise control of number of points in a chunk, and testing
  multiple sizes in one run.
- the b.N mechanism is good to get long benchmark runtimes which
  improves the reliability of the results

raintank/metrictank,raintank/metrictank,raintank/metrictank,raintank/metrictank