Add serial murder data

[mpjashby]

This needs to be carefully chosen to minimise the risk that anyone taking the course might be distressed because they know someone involved (this has happened to me before). It needs a travelling offender. Only the necessary details of the crimes should be included and no details of the offender.

Possibly using the canal murders around Frankfurt between 1976 and 1983. Use data from the Wikipedia page (the German and English pages are virtually identical, but both have exact dates and locations) plus this record of some newspaper research done (translated from DE to EN) to confirm some of the locations.