Clean up the fitting_with_x_and_y_errors tutorial

1. Fix the mangled xerr/yerr in the plotting routine that caused an Error in matplotlib 3.6+
2. Switch to the bilby_mcmc sampler to avoid pip installation of nestle
3. Run over the example and check it still works

added Bug label

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approved this merge request

merged

mentioned in commit b364e538

changed milestone to %1.3.0