Growth rates example notebook and bug fixes

[kpentland]

In this PR we provide users with a new example notebook (number 10) on how to compute vertical instability timescales/growth rates and the associated quantities of an equilibrium.

The notebook:

• explains how FreeGSNKE calculates the deformable growth rate for a given plasma equilibrium.
• demonstrates different ways to visualise and plot (un)stable modes, Jacobian matrices, and how the plasma deforms under an unstable mode.
• explains how to calculate other rigid (non-deformable) metrics such as the Portone parameter.
• explains how to use different options to decompose passive structure modes (saving runtime).

Other updates include:

• squashing a bug that was returning two unstable modes under some parameter combinations (a pseduoinverse matrix approximation has been replaced with the true pseduoinverse calculation).
• replacing some numy.linalg.inv calculations (with numpy.linalg.solve) that were likely leading to numerical issues/instabilities.
• re-numbered the example notebooks to account for more than ten.

[nicamo]

Thanks @kpentland, this looks great

[kpentland]

This PR is now slightly outdated due to fixes made in #20.

I've merged the relevant updates from here into #20 (e.g. new notebook 10, new numberings, np.linalg.inv removals, etc).

Closing this now.