Prefer a more terse way of converting vectors to numpy arrays

[badisa]

• Originally went down the route of attempting to use py::capsule to avoid having to perform a memory copy (as described in https://github.com/pybind/pybind11/issues/1042#issuecomment-325941022), but think because we wrap them in std::array the vectors are cleaned up before the capsule triggers.
• Probably should look at py::capsule in the future as it could reduce the memory copying taking place in our wrapped code as well as reduce the amount of time we require the GIL (relating to #1370 )

[badisa]

Going to merge into #1370 as there are some additional changes that I think would be worthwhile.