COM Tagesvektor: Validate `tag` is always the begin of the German Stromtag or Gastag

[hf-kklein]

Therefore:

• Determine the Timezone of the Datetime, raise an error if it's unknown
• Calculate the UTC-Timestamp of the begin of the German Strom and Gastag for the respective tag
• Check that the (hour, minute, second) is correct, meaning it's the strom or gastag
• UTC datetimes should always pass