Compare scenarios

[robinhasse]

• I add an initial set of compare scenarios scripts. Until now, only few stock and flow variables are plotted.
• You can now pass a file path to a custom reporting template. If there is now file, it is expected that you refer to a internal mapping as before.
• I improve the messages and errors thrown when variables are missing that reportAgg expects. Before, there was a unmeaning madrat error given when you tried to aggregate an empty madrat object. This happens, if you try to report a missing set element explicitly. Now, this leads to NULL being reported.
• You can now pass a file path to a custom reporting template. If there is now file, it is expected that you refer to a internal mapping as before.

[ricardarosemann]

Thank you for this, especially also for the additional warnings, probably very helpful.
I only found one point that I'd like to be clarified, otherwise it's all looking good.