add data sets, suggest ideas to organize a data challenge

[imkemayer]

(DONE) Some websites with more datasets that we could add:

• https://sci2s.ugr.es/MVDM
• https://openmv.net/tag/missing-data
• https://sci2s.ugr.es/keel/missing.php#sub2

[njtierney]

For the data challenge, if we want to organise something like what they have done for the M4 challenge, we will need complete datasets with different/complex missingness patterns generated, so we can assess imputations, etc.