- is_vectorspace :
  test existence and non-zero value of the Type 'rank' static variable

- pTraits_rank :
  value of 'rank' static variable (if it exists), 0 otherwise

- pTraits_nComponents :
  value of 'nComponents' static variable (if it exists), 1 otherwise

- pTraits_has_zero :
  test for pTraits<T>::zero member, which probably means that it also
  has one, min, max members as well

Note that these traits are usable with any classes. For example,

  - is_vectorspace<std::string>::value  ==> false
  - pTraits_nComponents<std::string>::value  ==> 1
  - pTraits<std::string>::nComponents  ==> fails to compile

  Thus also allows testing pTraits_rank<...>::value with items
  for which pTraits<...>::rank fails to compile.

  Eg, cyclicAMIPolyPatch::interpolate called by FaceCellWave with a
  wallPoint.

     pTraits<wallPoint>::rank ==> fails to compile
     is_vectorspace<wallPoint>::value ==> false

GIT: relocate ListLoopM.H to src/OpenFOAM/fields/Fields (future isolation)