Getting datasets in a group

### What are you trying to do?
Getting a list of datasets from a group for looping.

### What did you do?
I believe it's possible to use a combination of the functions here:
https://github.com/gonum/hdf5/blob/master/h5g.go#L108

```go
num := f.NumObjects()
```

Then looping for the range of num and calling `ObjectNameByIndex` on each.

### What did you expect to happen?
A property in the Group struct here: https://github.com/gonum/hdf5/blob/master/h5g.go#L25

An array with each of the dataset names so that they can be looped and opened easily with a `f.OpenDataset("myname")`

### What version of Go, Gonum, Gonum/netlib and libhdf5 are you using?
1.10.2

### Does this issue reproduce with the current master?
Yes.

Start with the Group struct and related functions in h5g.go, especially NumObjects and ObjectNameByIndex. Determine how dataset names should be exposed for iteration and verify that callers can use the resulting names with OpenDataset; add or update tests if the repository provides coverage for Group behavior.

Written by the indexing model from the issue text.