Trans mult container

This PR adds Transmissibility container the EclipseState object. The intention is that the container should include the (combined) effect of the different transmissibility modifier keywords MULT(XYZ), MULTFLT and  MULTREGT. Currently it will only return TransmissibilityMultiplier == 1.0

I encourage the consumers to start using this container in opm-core immediately, and then we will gradually give it more interesting content than 1.0.