Update Graph class and add basic search algorithms

[eholum]

For #34.

A* and DFS both work,I tried to not completely obliterate the Graph class's interface so as to avoid having to make significant changes to RRT. The weirdest thing is having to have a dictionary of nodes to themselves to support finding actual node objects in the Graph. I guess this is why Rust has a borrow checker.

In summary,

• Adds hash and eq functions to the Node and Edge classes so that you can find them in the Graph given a robot configuration
• Enables writing and loading a graph from a human-readable file format
• Nodes are now a dictionary to enable lookups of the actual objects managed by a Graph object
• Neighbors are now a dictionary of nodes and their costs (aka distances)
• Ensures neighbors are removed when edges are deleted
• Adds a DFS and A* implementation for graphs, and basic unit tests

Might need to think about how to make this cleaner...

[sea-bass]

Mostly nitpicky comments about docstrings and names.

Implementation looks solid so far, and I can't think of anything that would impact downstream planning approaches that would build such a graph.

Looking forward to these graph visualizations in the PRM implementation! Do take a look at how the RRT one puts stuff into Meshcat.

[eholum]

Adjusted some things, and I think I got all of your very valid nits.