HS3 implementation

[jonas-eschle]

HS3

keep track of the status of HS3 implementation in zfit

Known issues

• only top-level components are collected in PDFs. This is because objects in zfit do not have a unique name so far and we cannot simply replace them down the line. We would need to create a unique name for all the objects and we would need to recursively retrieve all the pdfs -> V2
• space cannot be called axes since this is a reserved keyword in zfit