Add new datasets to `darts.datasets`

[hrzn]

It would be nice to have more datasets - in particular "parts" and "traffic" used in some of the Amazon research papers.

A good source: https://archive.ics.uci.edu/ml/datasets.php

• Traffic: UCI Machine Learning Repository: PEMS-SF Data Set
  https://archive.ics.uci.edu/ml/datasets/PEMS-SF