ENH: upgrade to use some C++17 constructs

- 'if constexpr (...)'
   * instead of std::enable_if
   * terminate template recursion
   * compile-time elimination of code

- use C++14 '_t', '_v' versions,
  eg, std::is_integral_v<T> instead of std::is_integral<T>::value

- std::begin, std::end, std::void_t instead of prev stdFoam versions

- provide is_contiguous_v<..> as short form of is_contiguous<..>::value
  with the additional benefit of removing any cv qualifiers.

ENH: include is_rotational_vectorspace trait

- tests for vector-space and nComponents > 1 (ie, not sphericalTensor)

ENH: improve robustness of pTraits_.. tests by removing cv qualifiers