Make it possible to load addons from scenario folders

[Mathias157]

Addon equations and _hooks are always loaded from base.
The scenario folder structure of Balmorel may cause energy system modellers to believe that it is also possible to change addons by adding an addons folder in a specific scenario folder.

So, change the loading of addons, so that it is first checked if the specific addons/addon_name folder exist in the current scenario, before checking in the base folder. This is at least relevant for the files:

• balmorelbb4.inc
• bb4datainc.inc