Calculate new categorical variable derived from `ozcbi`

[florianm]

• Extend add_ozcbi_forest to create new variables after the mutate step adding ozcbi. Update docstring, examples, vignette.

  %>%
  dplyr::mutate(
  burn_severity_category = dplyr::case_when(
      ozcbi <= 0.3 ~ 1L,
      ozcbi <= 1.3 ~ 2L,
      ozcbi <= 2.1 ~ 3L,
      ozcbi <= 2.8 ~ 4L,
      ozcbi > 2.8 ~ 5L,
      TRUE ~ 0L
    ),
  burn_severity_label = dplyr::case_when(
      burn_severity_category == 1 ~ "Unburnt",
      burn_severity_category == 2 ~ "Low",
      burn_severity_category == 3 "Medium",
      burn_severity_category == 4 ~ "High",
      burn_severity_category == 5  ~ "Very High",
      TRUE ~ "Unknown"
    ),
  burn_severity_colour = dplyr::case_when(
      burn_severity_category == 1 ~ "green",
      burn_severity_category == 2 ~ "orange",
      burn_severity_category == 3 "red",
      burn_severity_category == 4 ~ "darkred",
      burn_severity_category == 5 ~ "brown",
      TRUE ~ "grey"
    )
  )

• Change map_ozcbi_forest to use burn_severity_colour for the marker colour.
• Add test coverage. Mind the edge cases - did we get the boundaries right?

Heath OzCBI

Complete - tbd Partial - tbd Unburnt - tbd