Simplified json file format for graphs

[jvdwetering]

This fixes #243, #193 and means #123 can be closed as now all core features for multigraphs should just work.

The original json format was inherited from Quantomatic, which is no longer in development or used, so using this format has no added use. In this pull request I build a method to convert Graphs to dicts and back in a simple way. The format is as follows:

{version: int, #to be future proof in case we add new features, so we know how to load the format.
backend: str, #which backend the graph should be loaded
variable_types: dict[names,0|1],
name: str,
scalar: str representation of Scalar,
inputs: list[int],
outputs: list[int],
auto_simplify: bool, # to make multigraphs load the correct value for here and prevent parallel edges from being removed.
vertices: list[VertData],
edges: [source,target,edgetype]
}

The VertData is as follows:

VertData = {
id: int, # Specifies what the index is of the vertex so that we preserve all the indices when we save and load, implements #193 
t: VertexType,
pos: [float,float],
phase: str,
data: dict[str,Any], # Contains all the g.vdata(v,..)
is_ground: bool # Whether there is a 'ground' generator attached to this vertex. Only used in mixed quantum/classical vertices
}

Because the version is now remembered in the json, I interpret a lack of a version in the dict as meaning it was saved in the old format. The legacy code is still there and hence can still be loaded.