feat: implement silv_density_sdimax and add sdimax_models dataset

[aitorvv]

Description

This PR implements the calculation of the Maximum Stand Density Index (SDImax) based on the models from Rodríguez de Prado (2020).
It adds the sdimax_models dataset containing the reference ("basic") and climate-dependent parameters, and introduces the new silv_density_sdimax() function.

Verification

# Inspect new dataset
devtools::load_all()
str(sdimax_models)   # 88 rows x 13 columns
dplyr::count(sdimax_models, model_name)  # basic: 15, and other climate-dependent models

# Verify new function - Reference (basic) model calculation
# Expected value for Pinus sylvestris: ~1114.77
sdimax_ref <- silv_density_sdimax("Pinus sylvestris")
print(sdimax_ref)

# Verify new function - Climate-dependent model calculation
# Expected value for Pinus canariensis (P1 at 400): ~103600.4
sdimax_clim <- silv_density_sdimax("Pinus canariensis", climatic_model = "P1", clim_value = 400)
print(sdimax_clim)