Fixing small bugs and improving output

[rabrie]

• Fixed GHZ plot showing wrong qubit counts
• Add GST config variable to pass to run_MGST(..., testing=) for plots
• In mgst.algorithm at line 721 add success=1 so that correct message is printed if the threshold was not reached but the alg. converged (with model mismatch etc.)
• Add note about the GST optional dependency in the installation guide somewhat prominently and also in the GST example notebooks.
• mGST docstrings in low_level_jit are wrong: "derivative by dK" instead of "derivative of dK" and so on
• Add gate labels qubit indices to GST gate labels in the compressive_gst.py
• GST saves an empty "y" in init
• Transpiled and untranspiled circuits were swapped in circuit class change for GST
• GHZ Fidelity plot: Adding grid, better x-labels, backend and timestamp in title, consistent coloring
• Added remaining GST print statements to log

[pedrofigro]

I only noticed little issues in docstrings for the GST files (some parameters missing, for example), but this doesn't have to be resolved in this PR. I'll continue reviewing the checklist points.

[pedrofigro]

The GHZ plot now shows the correct number of qubits! But the labels are funny, showing halves, for example. It'd be nicer to show labels only on the qubit counts of the experiment and a grid in the background. I also generally prefer seeing other information in the plot like backend name and time/date of the experiment.

[pedrofigro]

Approving now, given that the issues with the new circuit container are critical; can address the smaller comments in a separate PR.

[rabrie]

The GHZ plot now shows the correct number of qubits! But the labels are funny, showing halves, for example. It'd be nicer to show labels only on the qubit counts of the experiment and a grid in the background. I also generally prefer seeing other information in the plot like backend name and time/date of the experiment.

This is now included and also the plot colors are adapted to be in line with RB plots.
Example:

[pedrofigro]

neat! I've seen the prints changed to loggings now too, things look good from my side