[BUG] Clean up and improve pixel queue code

[masadcv]

Describe the bug

Clean up and improve code for pixel queue method as described in the comment here:

For clarity, it may be good to clean up a bit the pixel queue implementation. Maybe adding some comments to refer back to the algorithm described in https://doi.org/10.1016/j.patrec.2006.10.010

Also, I would replace the hand-crafted heaps by a standard use of a std::priority_queue: https://en.cppreference.com/w/cpp/container/priority_queue

[tvercaut]

The state values should also be changed to enum for clarity:

FastGeodis/FastGeodis/geodis_fastmarch.cpp

Lines 178 to 180 in c8c51a9

	auto state = torch::zeros({height, width}, options);
	// state value: 0 == accepted, 1 == temp, 2 == far away

[masadcv]

Also add better unittests as indicated here: #42 (comment)
And also include pixelqueue method in all sample code provided.