- construct from dimensioned/value, defaulting name from value.
  Can be convenient for these type of operations:

      max(.., dimensionedScalar(somedims, 0.5))

- construct from dimensioned/one, forwarding to pTraits::one.
  Can be convenient for constructors:

    volScalarField( ..., dimensionedScalar(somedims, one{}))

ENH: minor updates to zero/one classes.

- add global 'One' constant for symmetry with 'Zero'.