Add exact vectorization

[AlexeyTrekin]

Current vectorization is based on OpenCV and produces a simplified output.
It can lead to errors due to self-intersection of the geometries, and can cause holes and intersections in case of two near objects.
Also, we want a topojson-like structure if the objects of two different classes have the same boundary, but currently their boundaries will be different due to separate vectorization.

So what we need is:

• test if the current variant can make the exact vectorization if the parameters are correct ( probably not).
• vectorization by the actual pixel boundaries (like rasterio.features.shapes(), but with contour hierarchy).
• merge of the polygons that are connected by the corner, without self-intersection
• simplification of the contours that will preserve correctness of the shapes

The next step will be a topojson extension based on the current result.

Illustration: current result of the vectorization of two objects with the shared boundary - both intersection and gap are present.