update-mcp: Change when examine_neighbors is called.

Also renamed Mcp_Connect to MCP_Connect.

Makes more sense to call it when the neighbor is frozen. In that
way the method is called exactly once for each pair of neighbours.
Plus the costs and paths are known and fixed.
update-mcp: add new MCP classes to skimage.graph namespace
update-mcp: added tests for newly added features.

Also fix in existing test related to how the traceback arrays is initialized:
first it was initialized with all -1, now with -2, and -1 at the seed points.