Visualizing Quantum State (for Linux)

[indisoluble]

What

This PR continues the work done in New playground: Visualizing Quantum State. While the latter added visualization capabilities for macOS/iOS, this one does the same for Linux.

How

The new code in this PR relies in 2 different external dependencies:

• LAPACK
• WebKitGTK+

Although this PR is fully functional, the dependencies are imported with Swift Packages that are not part of QISKit organization. Until this is fixed, this PR has to be kept on hold.

FTR, the next LTS of Ubuntu will bring back GTK+ as main window manager, so to depend in WebKitGTK+ should not be troublesome (I think). About LAPACK, this is in fact a dependency of the C++ simulator, which as far as I know, will eventually be integrated in this Swift repo, so I guess it should be troublesome either.

Notice

The demo app for Linux relies on SPM to import the QISKit framework and, although the package is included with a relative path, it is mandatory to specify a tag. What I am trying to say is that, in order to test the app, if you download my own repo you will have to tag the last commit in VisualizingQuantumState branch with 0.1.0; if you merge this PR in the main repo, you will have to tag the last commit with the former tag afterward, otherwise swift build will fail.