FusionTree productspace layer

[Jakob-Unfried]

Implement combine/split for FTBackend

• Wait for Refactor spaces #26 to merge
• Strategy: combine does not change the underlying data at all, only the legs of a tensor are changed
• make sure combine and split do exactly this
• most other functions that do something with legs should be adjusted to deal with pipes. make a list which functions are affected -> subissues?
• reactivate any tests that are skipped for this reason
• check the helper methods in TensorProduct (custom iterators), do they work with pipes?
• check all usages of TensorProduct.factors, we should probably access a flat list of ElementarySpace instead via some combination of helper functions