Minor improvements

[kavanase]

This PR adds a few minor improvements to the plotting functionalities of CarrierCapture.jl:

• Adds the wf_sampling_rate option; for reducing the number of vibrational states plotted in the CC PESs. This makes them easier to visualise, and greatly reduces the output image file size and times to generate plots (often quite slow).

• Adds the alpha option, to adjust the opacity of the vibrational states to improve visualisation.

• Adds the linealpha option, to adjust the opacity of the PES lines and datapoints.

These are particularly useful when plotting some metastable PES states alongside, for instance. Example:

• Updates the get_eig.py script to clean up a little, improve visualisation by changing the colours (blue = occupied, orange = unoccupied and green = partially occupied, following the format of the eigenvalue plots in doped, and automatically save the plot to file:
  

[aronwalsh]

Thanks for this @kavanase. The CI error seems to be an extra ) on Line 20 of Plotter.jl. Then it should be good to go!

[kavanase]

Yep good thing there's CI on this! 😅
Typo fixed now 🤞