Bug fix where `pesummary.core.file.mcmc.burnin_by_step_number` was not removing samples as burnin
The purpose of this MR is to fix the pesummary.core.file.mcmc.burnin_by_step_number method to remove all samples with step number < 0 as burnin.
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The purpose of this MR is to fix the pesummary.core.file.mcmc.burnin_by_step_number method to remove all samples with step number < 0 as burnin.