Incorporate CO₂ capture retrofit in other nodes

You can find the mathematical description of the incorporation on the CO₂ capture retrofit page. The mathematical description explains as well the required changes to a RefNetworkNode to allow for incorporating CO₂ capture retrofit. All created methods for the existing functions are however created for the abstract supertype NetworkNodeWithRetrofit Hence, it is also possible for the user to create separate nodes that allow for CO₂ capture retrofit.

Creating a new type for retrofit, that is, e,g,, with differing additional constraints is in general simple. The new type only has the following requirements:

Is has to be a subtype of NetworkNodeWithRetrofit. This implies that said node is a NetworkNode, although we are currently thinking about changing the overall structure.
The type must have either a field called co2_proxy or you have to add a method to the function EnergyModelsCO2.co2_proxy of EnergyModelsCO2.
CO₂ capture data has to be incorporated as one of the CaptureData subtypes as described on Data functions.
If you have created a new method for EnergyModelsBase.create_node, you have to include in said method the following code lines
```
for data ∈ node_data(n)
    constraints_data(m, n, 𝒯, 𝒫, modeltype, data)
end
```
and the node must have either a field called data or you have to add a method to the function EnergyModelsBase.node_data to be able to access the data.
Alternatively, you can also manually extract the capture data and call directlty the functions constraints_data