Cs projection

Enhance the setup of the operators in the TimeDependenCorrelation class. Allow a similar
dictionary configuration for ``operator_t`` as already for ``operator_t0``.
It is now possible to arbitrary product operators as ``operator_t0`` *and* ``operator_t`` acting on multiple sites,
previously ``operator_t`` was restricted to be an onsite operator. Furthermore the configuration of ``operator_t``
can now be dropped, in which case the hermitian conjugate of the ``operator_t0`` is used.
If both operators act on multiple sites, correlations are still calculated by sliding the ``operator_t`` through
the MPS winding (starting at site 0) and by moving one site to the left. It is not yet supported to only calculate
the overlap on every n-th site.
Include a 'correlation_function_key' to name the measurement result accordingly.
Furthermore add "mixed-space" operators as (experimental) feature (with a more rudimentary setup for the operator
configurations). Update tests accrodingly.
Allow the spatial fourier transform to take an axis in spectral_function_tools.py and include a mixed_space parameter.

This is done by introducing the boolean parameter ``insert_JW_from_left``in
``expectation_value_multi_sites`` and ``_corr_ops_LP``

…into spectral_functions

…ndentCorrelation

Furthermore allow to pass shifted cylinder boundary conditions in Model (as integer)

Include the conversion to an integer in the try clause,
otherwise return zero

Convert the center of the lattice to an mps index before applying it.
Furthermroe enhance documentation for class `TimeDependentCorrelation`

…into spectral_functions