Can't open attributes on the root group

### What are you trying to do?
View an attribute on the root path of the HDF5 File "/"

### What did you do?
```
f, err := hdf5.OpenFile(event.Name(), hdf5.F_ACC_RDONLY)
if err != nil {
log.Fatalln("Panic, could not open file")
}

d, err := f.OpenGroup("/")
if err != nil {
log.Fatalln("Panic, could not open dataset")
return
}

attr, err := d.OpenAttribute("file_version")
```
OpenAttributes on a group doesn't seem to work. You can however call CreateAttribute, just not read from it.

### What did you expect to happen?
The ability to open an attribute like on a dataset.

### What version of Go, Gonum, Gonum/netlib and libhdf5 are you using?

go version go1.10.2 linux/amd64

### Does this issue reproduce with the current master?
Unknown