Add an absorbed power fraction to the generic heat source description

[anteplu]

• the input (injected) power is prescribed in the config;
• an absorbed power fraction has to be added to the config;
• the generic heat source module has to be updated such that the heat transport equations use the absorbed power rather than injected one;
• in the output both injected and absorbed powers are somehow available;
• the fusion gain Q in the output is computed using the injected power.