multiphaseEuler::phaseModel: Changed the phase-fraction field construction to work around a problem with Clang

The Clang compiler does not use std::move to transfer the result of the ternary operator into the phase-fraction field resulting in it not being registered to the database. To work around this limitation/bug the ternary operator is now provided with tmp fields the result of which is passed with an IOobject to the final field constructor to ensure it is registered and the IO options set correctly.