Storage duration about flextool

[irenaflextool]

In FlexTool3 storages are done with two (or three) objects: the storage node and the charger/discharger. The latter can be presented with a connection or with a unit. In my view, the best thing is to expand the constraint concept to include capacities. One will be able to fix either of those storage options, but there will also be further flexibility (when defining more complicated processes e.g. on industrial demand side).

I will need to

• rename constraint_coefficient to constraint_flow_coefficient
• add it to connection__node__node
• add parameters for constraint_capacity_coefficient (in unit__node, connection__node and in node)
• add constraint_state_coefficient to node

[jkiviluo]

This has been implemented a while ago.