Make networkx dependency optional

[MariusWirtz]

Switch verify_edges default to False

Fixes #1300

[onefloid]

I hate to ask it, but should we deprecate this first?

This isn’t a real breaking change, but rather a behavioral change with lower risk.

A gradual change could make sense here because the check only affects early error detection, not core functionality. Keeping it temporarily enabled helps avoid surprising users while still allowing the dependency to become optional later. Depending on how heavy the workflows are, it might still be beneficial to fail early to save resources. Also, if TM1 declines building the hierarchy, it should ideally raise the same ValueError; otherwise, the risk of breaking existing code increases due to differences in error handling in the calling code.

[vmitsenko]

Hi both, just a suggestion — it might be worth replacing the networkx library with a depth-first search implementation to maintain backward compatibility and simplify dependencies.