Footnotes

• Contributors RG-S conceived the presented idea. RG-S and IGF designed the method of the analysis. RG-S wrote the first proposal after PL verified the analytical methods and supervised the findings of this work. All authors discussed the results and contributed to the final manuscript.

• Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.

• Competing interests None declared.

• Provenance and peer review Not commissioned; internally peer reviewed.