constructors of SIMDWrapper types

The following discussion from !2257 (closed) should be addressed:

• @graven started a discussion:

  actually, why not add a constructor to dType::float_v which takes either a(reference to a)n array or span of floats, with size dType::size in which case one would be able to simply write:

          return LHCb::Rec::Chi2PerDoF<dType>{typename dType::float_v{tmp_f}, typename dType::int_v{tmp_i}};

  which would give some extra compile time checking that the sizes match?

(where tmp_f is a std::array< … >)

• span
• std::array