Drop QuTiP dependency

[thangleiter]

In the core package, QuTiP is only used for plotting Bloch sphere trajectories. Reimplementing this (or copying their code should the licensing allow it) should significantly lower the installation barrier.

At first glance, this would require

• reimplementing plotting.plot_bloch_vector_evolution() and
• dynamically checking if QuTiP is available for the types.Operator, types.State types.

Keeping the dependency for the examples / documentation is okay since we still want to interface easily with QuTiP.

[thangleiter]

Much simpler than reimplementing plotting.plot_bloch_vector_evolution() etc. is simply handling QuTiP as an optional dependency at install. Hence, we'd introduce a new extra, bloch_sphere_visualization with requirements matplotlib and qutip.

In the same vein, it makes sense to make the whole plotting module an extra. It is not part of the core functionality and requires an additional dependency (matplotlib). Since most users will have matplotlib installed anyway, the module should be available to mostly everyone anyway.