This function iterates through a list of models and extracts year and associated measurements.
     
    
    Usage
    .get_bts_df(M, biomass = TRUE)
 
     
    
    Arguments
- M
 
A list of models where each model has the needed attributes.
 
- biomass
 
Logical indicating if biomass data should be used. If FALSE, other measurements are used.
 
 
    
    Value
    A data frame with columns for year, model, observed values, predicted values, lower bounds, and upper bounds.