Measurement function fixes

[jhauschild]

Again, this change is backwards incompatible, so I'm creating a pull request to make the change more visible and track it better.

The relevant incompatibility is in f580506 (cherry-picked from 2a5052a), namely that measurement functions need to accept models as additional arguments. This is necessary to ensure compatiblity of the psi to be measured with the model, e.g. when grouping sites.

While you're changing those measurement functions, you might also want to follow the new TeNPy convention that they start with an m_, e.g. we renamed tenpy.simulations.measurement.bond_dimension to tenpy.simulations.measurement.m_bond_dimension. For now, the old names still work in the config/yaml files, but they give a FutureWarning.
You should update the config yaml files in projects where you want to update to a new TeNPy version:

• update the connect_measurements simulation parameter: all direct measurement functions (taking results, psi, model, simulation as arguments and saving to results) defined in TeNPy have been renamed to start with m_.
  At this point, all those functions are defined in tenpy.simulations.measurement.py.
• If you're using psi_method to measure e.g. correlation functions, you need to update the entry as follows:
  An old python entry for connect_measurements

{...,
 'connect_measurements': [['tenpy.simulations.measurement',
                           'psi_method',
                           'correlation_function',
                           {'key': '<Sp_i Sm_j>', 'ops1': 'Sp', 'ops2': 'Sm'},
                          ],
                          ...
                          ]

can get replaced with new entry:

{...,
 'connect_measurements': [['psi_method',
                           'wrap correlation_function',
                           {'key': '<Sp_i Sm_j>', 'ops1': 'Sp', 'ops2': 'Sm'},
                          ],
                          ...
                          ]

Similarly, an old yaml entry for connect_measurements

connect_measurements:
    - - tenpy.simulations.measurement
      - psi_method
      - method: correlation_function
        key: '<Sp_i Sm_j>'
        ops1: Sp
        ops2: Sm
    - ...

can get replaced with new yaml:

connect_measurements:
    - - psi_method
      - wrap correlation_function
      - results_key: '<Sp_i Sm_j>'
        ops1: Sp
        ops2: Sm
    - ...

[jhauschild]

Further information can also be found in the new measurement user guide