Multiple issues with mixing coefficients

[tchipev]

• misleading warning "This can happen because the xml-input doesn't support these yet!"
• neededCoeffs check should be multiplied by two "if(dmixcoeff.size() < neededCoeffs){" and this should probably be an ==
• duplication between Domain::_mixcoeff and EnsembleBase::_mixingrules
• special mixing rules with xi and eta are not symmetric, but Comp2Param::initialize is symmetric?
• appending values at the end of _mixcoeff in Simulation.cpp:250 error prone
• initialization is dependent on the order in which they appear in the file and the passed componentid-s have no effect ?

[tchipev]

all except duplication between Domain::_mixcoeff and EnsembleBase::_mixingrules should be fixed with Pull Request Nr #76