BUG: TES.ddck is exported when nothing is connected

[ahobeost]

As a teacher, it would be helpful for certain User mistakes to be caught by the system.
This would allow the GUI to provide helpful feedback and prevent larger issues occurring later on.

Currently, the TES.ddck can be exported when no ports and no HXs are connected.
Instead, a message should pop up stating something like:
"A storage without input and output is useless. Please add port pairs and/or internal heat exchangers."

[zuckerruebe]

Are connections baked into the exported ddck or dealt with in the "placeholders JSON" file? If the latter, I'd argue that the ddck can "stand on its own", similar to other ddcks. If the former: totally agree.

[ahobeost]

They are baked into the exported ddck.
Because the TES is often unique for each project (changing nr of ports/hxs, changing locations of same), it does not make sense to use the placeholder values in this case.