co2: generate a simple data set 
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 Based on the data set related to CO2
    https://github.com/owid/co2-data/raw/master/owid-co2-data.csv
    https://github.com/owid/co2-data 

 Generate a data set with just 4 columns:
    iso_code,country, year and share_global_c02, among the total variables. 

> print(d,quote=F)
 [1] iso_code                            country                             year                               
 [4] co2                                 consumption_co2                     co2_growth_prct                    
 [7] co2_growth_abs                      trade_co2                           co2_per_capita                     
[10] consumption_co2_per_capita          share_global_co2                    cumulative_co2                     
[13] share_global_cumulative_co2         co2_per_gdp                         consumption_co2_per_gdp            
[16] co2_per_unit_energy                 coal_co2                            cement_co2                         
[19] flaring_co2                         gas_co2                             oil_co2                            
[22] other_industry_co2                  cement_co2_per_capita               coal_co2_per_capita                
[25] flaring_co2_per_capita              gas_co2_per_capita                  oil_co2_per_capita                 
[28] other_co2_per_capita                trade_co2_share                     share_global_cement_co2            
[31] share_global_coal_co2               share_global_flaring_co2            share_global_gas_co2               
[34] share_global_oil_co2                share_global_other_co2              cumulative_cement_co2              
[37] cumulative_coal_co2                 cumulative_flaring_co2              cumulative_gas_co2                 
[40] cumulative_oil_co2                  cumulative_other_co2                share_global_cumulative_cement_co2 
[43] share_global_cumulative_coal_co2    share_global_cumulative_flaring_co2 share_global_cumulative_gas_co2    
[46] share_global_cumulative_oil_co2     share_global_cumulative_other_co2   total_ghg                          
[49] ghg_per_capita                      methane                             methane_per_capita                 
[52] nitrous_oxide                       nitrous_oxide_per_capita            population                         
[55] gdp                                 primary_energy_consumption          energy_per_capita                  
[58] energy_per_gdp                     

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