STYLE: more consistent use of dimensioned Zero

- when constructing dimensioned fields that are to be zero-initialized,
  it is preferrable to use a form such as

      dimensionedScalar(dims, Zero)
      dimensionedVector(dims, Zero)

  rather than

      dimensionedScalar("0", dims, 0)
      dimensionedVector("zero", dims, vector::zero)

  This reduces clutter and also avoids any suggestion that the name of
  the dimensioned quantity has any influence on the field's name.

  An even shorter version is possible. Eg,

      dimensionedScalar(dims)

  but reduces the clarity of meaning.

- NB: UniformDimensionedField is an exception to these style changes
  since it does use the name of the dimensioned type (instead of the
  regIOobject).