Enhance OrthogonalExcitations

[jhauschild]

The OrthogonalExciations allows to convert an initially infinite MPS to segment boundary conditions.
However, as of now, it always uses the same set of left and right environments.
It should be fairly straightforward to adjust the extract_segments_from_infinite method to support

• topological excitations between different left/right environments from different degenerate ground states
  (e.g. for the transverse field ising chain, a segment between up on the left and down on the right)
• studying half-infinite chains where one environment is taken to be trivial.
  See https://github.com/tenpy/tenpy_notebooks/blob/main/14_segment_half_infinite.ipynb.

[jhauschild]

This was implemented in #266 (except that we should maybe add an example and/or more documentation on how to do this...)