Use the default time pattern for persisting time series keys instead of the `toString` method

[t-ober]

Currently the to string persistence outputs time strings with the [UTC] postfix which we can't easily parse. Use the default time pattern instead.

[t-ober]

Also I decided to simplify the time pattern to conform to the default time pattern we set up in the TimeUtils of the PowerSystemUtils

[t-ober]

@sebastian-peter : This is the discussion we had some time ago regarding ZonedTimeStamps I started a branch. But changing the toString method also changes the result output format (removing the [UTC] part). I'm willing to bite the bullet and change all the tests but first I wanted to get your opinion.

The logic changes are quite minor, have a look here: #767

[t-ober]

As per our discussion:

1. Long term we would prefer LocalDateTime to avoid confusion and since we don't expect different time zones.
2. Short term we might use UTC therefore adding the Z to the time pattern.
3. Check time zone of our weather data.

For the short term solution we need to make sure that we actually respect the time zones in weather retrieval accordingly!!

[danielfeismann]

Additional it might be also necessary to check PvModels needs of time format - e.g. in calcHourAnlgeOmega.