Remove deprecation warning from extended source stencil

/builds/dumux-repositories/dumux/dumux/multidomain/embedded/couplingmanager1d3d_average.hh:138:9: warning: ‘decltype(auto) Dumux::CouplingManager<Traits>::curSol() [with Traits = Dumux::MultiDomainTraits<Dumux::Properties::TTag::TissueBox, Dumux::Properties::TTag::BloodFlowBox>]’ is deprecated: This function returns a Dune::MultiTypeBlockVector<SubDomainSolutionVector&, ....> (i.e. storing references). Use curSol(domainIdx) to get a reference to the corresponding subdomain solution vector. Will be removed after 3.5 [-Wdeprecated-declarations]
  138 |         extendedSourceStencil_.evalAdditionalDomainDerivatives(*this, domainI, localAssemblerI, this->curSol(), A, gridVariables);
      |         ^~~~~~~~~~~~~~~~~~~~~~

The extended source stencil receives this->curSol() but this is actually not necessary since it already gets *this. Internally it currently only uses the solution of domainI which can be obtained from the couplingmanager.

TODO

• deprecated old interface
• implement interface without curSol