dubious combineFields operation in fieldValue

@andy, @Sergio

1. Write access of tmp using (), but even with ref() it wouldn't be clearer about how/where data are managed - code isn't called anywhere anyhow.

    void combineFields(tmp<Field<Type>>& field)
    {
        combineFields(field());
    }

2. combineFields does a gather/scatter + combine. However, the only consumer at the moment seems to be a surfaceWriter, which channels everything via the master. Could perhaps reduce some communication here and also micro-optimize for serial case.