Use new shift inversion methods in instrument class

Note results can change within the inversion tolerance. The reason is that
each dask chunk is inverted separately, which may lead to fewer iterations on the
faster converging chunks of data.
The boundary conditions during the interpolation within the fixed point iteration are
flat. All points are used in the convergence measure. In contrast, the old scheme excluded
a hardcoded margin.

TODO: the case where interpolation is set to None is currently not implemented.

All unit tests are passing.