dimensionedType: Extend lookupOrDefault to handle any form of dimensioned type input

e.g.
     Not specified: default used.
     Fully specified: rhoMax          rhoMax [ 1 -3 0 0 0 ] 2.0;
     Without name:    rhoMax          [ 1 -3 0 0 0 ] 2.0;
     Value only:      rhoMax          2.0;