expose LineSegmentDetector to Python

- exposed LineSegmentDetector class to python
- renamed createLineSegmentDetectorPtr -> createLineSegmentDetector

There are a some issues I want to discuss:
- currently LSD works in LSD_REFINE_ADV mode only due to (lsd.cpp:427)

```
    CV_Assert((!_nfa.needed()) ||                              // NFA InputArray will be filled _only_ when
              (_nfa.needed() && doRefine >= LSD_REFINE_ADV));  // REFINE_ADV type LineSegmentDetectorImpl object is created.
```

I think we need some way to specify arrays which are "needed" from python side. Quick and dirty fix is just to comment the assert out.

- LSD returns integer segment coordinates, although it uses doubles internally. Shouldn't we return floating point coords to keep subpixel positions?