Convenience graph creators

[razziel89]

Creating regular graphs is not too easy right now. It would be cool to be able to create graphs from regular grids with minimal commands.

Features:

• Provide grid size in all dimensions
• Provide metric function used to determine whether two nodes are neighbours (requires iteration over entire grid for every point, which is inefficient)
• Provide list of displacements to reach neighbours (remember to track node positions in the creation algorithm, this is very similar to the example in the readme)
• Take function that retrieves cost for each node, allow nil, in which case the cost will have to be set manually
• Take name creator function
• Make sure to add position as payload

[razziel89]

A 2-D graph creator will be added. And N-D version could be added once there are generics. For now, the function is straightforward to modify for other dimensionalities in case users require higher dimensions.

[razziel89]

Implemented via #26. Closing.