Features/#353 pypsa eur results in 3h resolution

[ClaraBuettner]

Fixes #353 .
Fixes #352

[ClaraBuettner]

I did a run in test mode (this one + Carlos branch) and it was working fine. I could not test all tasks since some of them are also not running on Carlos branch, but at least there are no issues in the ones there were running.
I added a small sanity-check to make sure that all final time series have 8760 entries, that one was successful. It is running automatically after each run, so it will be also helpful when we can run all tasks.
I also searched in all files where the network from pypsa-eur is used and added the functions prepared_network and read_network there. This way, we only need to update the paths in one file.
I also updated the paths to the prepared or solved network for the case that pypsa-eur was not executed.