[jhauschild]

Summary

This PR adresses the first part of #254: It renames the LabelledLegs class to LegLabels, and removes the LabelledLegs from the Tensor / TensorPlaceholder MRO, i.e. they are no longer subclasses, but now own a LegLabels labels instance member instead.
This comes with an few changes for the calling interface, as given below (and in docs/cpp_conversion/convert_LegLabels.md). I hope/think that these should make the calls actually more intuitive.

Moreover, I opted to replace the None standing for "no label set" with an empty string, i.e. non-set labels appear/need to be set as empty strings , unifying the type for the label list.

Call-site map (Python)

Old	New
LabelledLegs	LegLabels
t.labels / t.labels() as a list	t.labels (LegLabels); copy via t.labels.as_list()
t.set_labels(x)	t.labels.set(x) or t.labels = x
t.set_label(i, x)	t.labels[i] = x
t.relabel(m)	t.labels.update(m)
t.get_leg_idcs(x)[0]	t.labels.index(x)
t.get_leg_idcs([...])	t.labels.indices([...])
t.has_label(...)	t.labels.has(...) or 'x' in t.labels
t.labels_are(...)	t.labels.are(...)
t.is_fully_labelled	t.labels.is_fully_labelled
t._labels / t._labelmap	remove (compare t.labels or use as_list())
for x in t.labels / list(t.labels)	for x in t.labels.as_list() / t.labels.as_list()
assert t.labels == [a, None, b]	assert t.labels == [a, '', b]

Label values

Old	New
OptionalLabel / std::optional<std::string>	std::string
OptionalLabels	std::vector<std::string> (LegLabelList)
std::nullopt / Python None as unset	""
if (lab) / lab.has_value()	!lab.empty()

LegLabels supports equality, len, [] get/set (int or str key), and
assignment from a sequence. It does not implement __iter__.

relabel1 / relabel2 kwargs on compose / tdot / outer / etc. are unchanged
(function arguments, not tensor methods).

Chaining note: old t.relabel(m) returned self; t.labels.update(m) returns
the LegLabels member. Split chains:

# old: expect = move_leg(...).relabel({...})
expect = move_leg(...)
expect.labels.update({...})

HDF5

• Nested "labels" stores a string list; load old None entries as "".
• Standalone roundtrips: LegLabels::{save_hdf5,from_hdf5}.

Bindings / stubs

• pybind/tensors/py_labels.cpp: bind LegLabels; sequences of str only.
• Drop nested labels property and _labelmap from the old holder API.
• lasyten/_core.pyi: list[str], not list[str | None].

[jhauschild]

This is ready for merge, but requires downstream changes, see tenpy/tenpy#715