Fit settings and vignette update

[djmannion]

• Fits sigma via log sigma internally.
• Fits sigma_e via the log of the scaling factor of sigma internally.
• Scales the parameters to a common scale within the fitting algorithm.
• Increases the maximum number of potential optimiser iterations.
• Updates the vignette to warn about long runtimes and to encourage checking of the optimiser convergence value.
• Changed packageVersion to utils::packageVersion in plot.R because check() was complaining.
• Updated the plot test images, likely due to changes in the fitting generating different estimates.
• Minor stylr changes.

[mariadelmarq]

It all looks very sensible, thanks heaps!