Clarification of heat generation from synthetic fuels and biogas

[danielhuppmann]

I found two issues related to variables for heat generation:

1. There is no variable for heat generation from synthetic fuels (e-fuels) - should this be added? Or is this not included on purpose because this would be highly inefficient anyway?
2. The variable "Secondary Energy|Heat|Biomass" is currently described as "centralized heat from biomass, including biogas" - but this may not be clear from the variable-name and it may be interesting to distinguish between woody biomass versus biogas explicitly. Should this be changed?

Please advise, @IAMconsortium/common-definitions-coordination @IAMconsortium/common-definitions-energy