- The pTraits_cmptType returns the data type of 'cmptType' (for
  arithmetic and VectorSpace types) or is simply a pass-through.

  This can be combined with the pTraits_nComponents for casting.
  For example,

  function
  (
      reinterpret_cast<pTraits_cmptType<Type>::type*>(buf.data()),
      (buf.size()/pTraits_nComponents<Type>::value)
  );

ENH: extend Foam::identityOp so support array indexing (pass-through)