Improve dataset accessibility

[MartijnCa]

It would be nice to make at least one or two datasets publicly available within this project in a proper easy-to-use data format.

As a first step we need to inquire the Alliander data office whether we can opensource this data.

Acceptance criteria:

• data folder containing measurements and predictor (weather / market) data in same format with same index.
• Index is a properly formatted ISO 8601 datetime with Zulu notation (UTC)
• Index has no duplicates nor missing timepoints (full index)
• Missing data points are indicated as NaNs
• It is clearly documented whether 15 minute time-intervals are right bounded (01-15 -> 15) or left bounded (00-14 -> 00)
• Data has been open sourced if allowed by Alliander Data Office.