Allow rad69 request splitting

[Sjoerd Kerkstra]

• dicomtrolley version: 1.1.1.
• Python version: any
• Operating System: any

Typically a rad 69 request contains all slices that you want to download.
For ill-configured rad69 servers, asking for all slices at once might cause issues.
Additionally, the way the split is made could impact performance; splitting on series for example.

Make it possible to split requests in arbitrary ways implement chunk and series splitting.

[Sjoerd Kerkstra]

Fixed by 14eb3f4