ENH: Added function attribute to read_binary for valid dtypes

the context manager here is more-or-less
```
try:
    openedfile = open(..)
    data = np.readfile(openedfile)
finally:
    close(openedfile)
```

If the code continues through the context manager then none of the code inside has raised an exception and we have to assume that `data` exists and is filled properly.
```