Rename as QXGraphDecompositions

[brenjohn]

Summary

Update the package name to QXGraphDecompositions.

Rationale

The name change is required to avoid conflicts when automatically registering it as a Julia package.

Implementation Details

Additional notes

Closes #3

Check before merging:

• All discussions are resolved
• New code is covered by appropriate tests
• Tests are passing locally and on CI
• The documentation is consistent with changes
• Any code that was copied from other sources has the paper/url in a comment and is compatible with the MIT licence
• Notebooks/examples not covered by unittests have been tested and updated as required
• The feature branch is up-to-date with the master branch (rebase if behind)
• Incremented the version string in Project.toml file