Cotengra

- [``einsum``](cotengra.einsum), [`einsum_tree`](cotengra.einsum_tree)
and [`einsum_expression`](cotengra.einsum_expression): add support for all
numpy input formats, including interleaved indices and ellipses.
- remove some hidden `opt_einsum` dependence (via a `PathOptimizer` method)

- add [``ctg.array_contract_path``](cotengra.array_contract_path) for returning
the raw contraction path only, with caching. Add caching to
[``array_contract_expression``](cotengra.array_contract_expression) and
related functions too.
- fix [`tree.get_eq()`](cotengra.ContractionTree.get_eq) when the ``inputs``
are a tuple of `Sequence[str]` rather than a `str`.

- add [tree.contraction_scaling](cotengra.ContractionTree.contraction_scaling)
- add [get_symbol_map](cotengra.get_symbol_map)

- add [`einsum`](cotengra.einsum)
- add [`einsum_tree`](cotengra.einsum_tree)
- add [`einsum_expression`](cotengra.einsum_expression)
- add [`array_contract`](cotengra.array_contract)
- add [`array_contract_tree`](cotengra.array_contract_tree)
- add [`array_contract_expression`](cotengra.array_contract_expression)
- add [`AutoOptimizer`](cotengra.AutoOptimizer)
- add [`AutoHQOptimizer`](cotengra.AutoHQOptimizer)
- remove most hard dependencies (`numpy`, `opt_einsum`)
- update [`tree.plot_contractions`](cotengra.plot.plot_contractions)