ENH: support construct Geometric field from separate value/dimensions

- can be easier to specify than copying into a dimensioned type.

  Eg,
      volSymmTensorField(io, mesh, Zero, dimLength);

      fluxFieldType
      (
          io
          mesh,
          Zero,
          Uf.dimensions()*dimArea/dimTime
      );

  vs
      volSymmTensorField
      (
          io,
          mesh,
          dimensionedSymmTensor("0", dimLength, Zero)
      );

      fluxFieldType
      (
          io
          mesh,
          dimensioned<typename flux<Type>::type>
          (
              Uf.dimensions()*dimArea/dimTime, Zero
          )
      );