Support extraction of clusters from graph state

[king-p3nguin]

Before submitting, please check the following:

• Make sure you have tests for the new code and that test passes (run tox)
• format added code by black -l 120 <filename>
• If applicable, add a line to the [unreleased] part of CHANGELOG.md, following keep-a-changelog.

Then, please fill in below:

Context (if applicable):

86

Description of the change:

• Add Cluster class to define extracted cluster.
• Add function extract_clusters_from_graph which will extract GHZ and linear clusters from graphix.GraphState.

Related issue: Related to #86

also see that checks (github actions) pass. If lint check keeps failing, try installing black==22.8.0 as behavior seems to vary across versions.

[shinich1]

@king-p3nguin could you add API docs?

[shinich1]

Perhaps an example too?

[shinich1]

do we want to mention fusion-based QC ref or something similar in the docstring?

[king-p3nguin]

I made the following changes:

• Previous extraction algorithm did not consider isolated nodes in the graph state as a cluster. Now it does.
• Added API docs
• Added an example
• Mentioned [1] for the definition of fusion in the example. [1] Daniel E. Browne and Terry Rudolph. Resource-efficient linear optical quantum computation. Physical Review Letters, 95(1):010501, 2005.

[king-p3nguin]

I also modified some docs to resolve sphinx errors.

[shinich1]

@king-p3nguin looks good to me, please squash and merge! let's use commit message fusion network extraction for graph state generation (#87). after this is merged, I will change some names e.g. extract_clusters_from_graph because cluster state sounds like it's larger than graph states in general and extract_fusion_network may make more sense. what do you think?

[king-p3nguin]

@shinich1 Thank you for reviewing! I will merge this now. Come to think of it, as you mentioned, the word "cluster" may create confusion by making people think of cluster states rather than mini clusters in the graph state. Maybe get_fusion_network or get_fusion_network_from_graph_state will be nicer, since the size of the fusion network is similar to the graph state.

[shinich1]

@king-p3nguin your suggestions look good! I will make these changes and get new release out soon, so that we can put the new version in the graphix-Perceval's requirements.