Extracts biogas feedstock from a MAgPIE GDX file at ISO country level. Currently covers two sources: manure allocated to digesters and fodder crops (placeholder, all zeros until MAgPIE tracks foddr/forage crop allocation to biogas). Manure is converted from nitrogen mass (Mt N) to energy (PJ) using livestock-specific heating values and nitrogen contents from Hoyos-Sebá et al. (2024).