Attribute for existing capacities in process and transmission

[ojdo]

Transfer the capability for representing already existing infrastructure, just like in sister model urbs. Implementation: all process and transmission capacity variables must be doubled (cap_total and cap_new). Then, an attribute installed_capacity must be added to the relevant model entities, while adding a new constraint

cap_total = installed_capacity + cap_new

Investment costs then must only occur for cap_new, while all capacity-dependent fix costs must incur for cap_total. Caveat: possible complications with the binary decision variables must be thought through before writing code.