add fit(method="tree") and fix ALS for complex TNs

[jcmgray]

This PR adds the function

• tensor_network_fit_tree for fitting any tree structured TN to any other (not necessarily tree-like) TN. It uses the canonical form of the tree but no intermediates reuse which might be helpful e.g. if the target TN is also a tree.
• you can access it via tn.fit(target, method="tree")
• it also fixes the usual ALS fitting for complex TNs
• it also auto enables lazy tn linear operator iterative solving to allow faster fitting of much larger tensors in the ALS method.