Add method to numpy::dtype to set byteorder

I've taken a look at `numpy/dtype.hpp`, and there does not appear to be any way to set the byte order of the array being constructed. The workaround I am currently using (which requires adding the numpy C API to my build and including the headers in my source) is the following:

```
np::dtype scalarDtype = np::dtype::get_builtin();
reinterpret_cast(scalarDtype.ptr())->byteorder = '>';
```

So it should be as simple as adding a method to dtype to allow the user to set the byteorder on the underlying numpy struct.

Start in numpy/dtype.hpp and review the existing dtype methods and how the underlying NumPy descriptor is exposed. Add the requested byte-order operation without requiring callers to include the NumPy C API, then verify that the dtype's byte order changes as requested.

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