Storing convergence information within bru object in user-friendly way

[charles-cunningham]

Convergence tracking information is currently stored in bru object ( fit$iinla$track) in an inconvenient format. This could potentially be changed to a more practical format for users wanting to gain information on model convergence.

[finnlindgren]

There is now a function to plot convergence diagnostics in a relatively friendly way:

fit <- bru(...)
bru_convergence_plot(fit)

[finnlindgren]

The tracking info is now stored in fit$bru_iinla$track in a "tidy" format, and bru_convergence_plot(fit) has been improved.