Update entso-e dataframe result structure and add timezone awareness

Closes #167 (closed)

• Move resolutions from the rows to the columns. This simplifies accessing different resolutions for each node.
• Adds timezone awareness so the request is using the local timezone if no specific timezone is given

To simplify the addition of further endpoints resampling is done for every resolution and the resolutions are concatenated in the end. This ensures that each resolutions is resampled to the correct time interval.

The XML samples for testing are extended to allow the testing of the correct concatenation of multiple days and later usage of timezones.