Consider deprecations before 3.0.0

[BjoernLudwig]

When we are getting closer to the release of PyDynamic 3.0.0 we should consider the following deprecations:

• Parameter justFit in PyDynamic.model_estimation.fit_filter.LSIIR can be removed, since the same is achieved by setting max_stab_iter to 0. To warn about hte pending deprecation we could for example introduce

      if justFit:
          warn(
              "LSIIR: The parameter justFit is only available for reasons of backward "
              "compatibility. You seem to intend to skip stabilization. Please from "
              "now on use `max_stab_iter = 0` in that case and leave `justFit` "
              "untouched. `justFit` will be removed in a future release.",
              PendingDeprecationWarning,
          )
  

  at the very beginning of PyDynamic.model_estimation.fit_filter.LSIIR like in the intermediate commit for release 2.0.0
  d4a6fc6 some time before the actual release.

• The functions invLSIIR and invLSIIR_unc in PyDynamic.model_estimation.fit_filter which we kept for reasons of not introducing another breaking change during the release of 2.0.0. For a start we could introduce at the beginning of PyDynamic.model_estimation.fit_filter.invLSIIR

      warn(
          "invLSIIR: The method invLSIIR() will be removed in the next major release "
          "3.0.0. Please switch to the current method LSIIR() with the same input "
          "parameters as in your call of invLSIIR in the same order and "
          "additionally specify inv=True.",
          PendingDeprecationWarning,
      )
  

  like in the intermediate commit d4a6fc6.