Improve and simplify loading of transformation chains

[SimonHeybrock]

See docstring in nxtransformations.py for an explanation.

I have added class DependsOn which seems a much saner way to handle the references, instead of juggling absolute and relative paths. The subclass TransformationChain additionally hold the chain of transformations starting at the depends_on.

Fixes #234 by introducing a new mechanism that can work around the shortcomings of the default data load. See there also for larger context.

Technically this represents a breaking change, since users would have to call build() after loading groups with transformations. In practice this is however typically not accessed directly — users typically use snx.compute_positions instead of working with the raw chains.