Implementation of Censure(STAR) Feature Detector

### Issues to address:
- [x] Implement STAR filter
- [x] Speed. Compare with numpy convolve?
- [x] Check the dependency of `rpc_threshold` with block_size at various scales
- [x] Decide the size of Octagon filter for scales > 7
- [x] Wrap lines and other PEP8 changes