Detect more square patterns

As the one found in the benchmark `slowparts.py'
Slightly faster implementation of the noffset function

Avoid passing the whole ndarray as parameter when only its shape
matters.
Constant reciprocal optimization

Turn a / cst into a * (1. /cst), as found in `growcut.py'
Merge pull request #349 from serge-sans-paille/bugfix/numpy-power-speed

Detect more square patterns