Documentation improvements

[aitorvv]

Description

This Pull Request standardizes documentation across the core forest metrics modules (predict-height, predict-biomass, predict-carbon, and predict-snfi-volume). It ensures precise unit reporting (diameter/dnm in cm, height in m, biomass/carbon in kg, and volume in dm³) and provides detailed, dataset-based tutorial examples for applying these functions directly to forest inventories.

Additionally:

• Height prediction default: silv_predict_height() now defaults to the generic eq_hd_vazquez_veloso_2025("All the species") if no model is specified.
• Young plantations support: Expanded silv_predict_biomass() to support root collar diameter (rcd) and biomass packing (bp) for young plantations (Menéndez 2022) when DBH/diameter is absent.
• S7 plot consistency: Merged class-specific plot documentation into the plot generic's page via @rdname plot and aligned generic signature fields to prevent R CMD check codoc mismatches.

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Verification

You can dynamically explore the help pages and verify the logic with the following executable R code:

# 1. Load package
devtools::load_all()

# 2. Explore updated documentation interactively
?silv_predict_height
?silv_predict_biomass
?silv_predict_carbon
?silv_predict_snfi_volume

# 3. Verify height prediction (with default model)
heights_default <- silv_predict_height(diameter = c(20, 25, 30))
# heights_default must be: 14.07234 15.90694 17.50925 (meters)
print(heights_default)

# 4. Verify young plantation biomass prediction (Menendez 2022) using rcd only
model_menendez <- eq_biomass_menendez_2022("Pinus pinaster")
predicted_young <- silv_predict_biomass(
  rcd    = c(5.2, 7.1, 9.4),
  height = c(2.1, 3.2, 4.5),
  model  = model_menendez
)
# predicted_young must be: 2.152248  4.972754 10.253238 (kg)
print(predicted_young)

# 5. Verify carbon prediction auto-selection and fallback
carbon_data <- silv_predict_carbon_auto(
  biomass   = c(100, 150),
  species   = c("Pinus pinaster", "Pinus radiata"),
  component = "stem"
)
# carbon_model must fallback to "montero-2005 (genus fallback)" for Pinus radiata
print(carbon_data)

# 6. Verify SNFI Volume calculation (mix of provinces/species names)
vol_results <- silv_predict_snfi_volume(
  province = c("Cantabria", "Araba/Álava"),
  species  = c("Pinus radiata", "Pinus sylvestris"),
  dbh      = c(20.5, 18.2),
  h        = c(15.2, 12.0)
)
# vol_results must return valid volume predictions in dm^3
print(vol_results)