h5ls: Ability to print attribute values without printing dataset values

[lamm45]

When using h5ls, I would like to be able to print attribute values without printing other data values.

In some earlier versions of h5ls (maybe the one that was included in Ubuntu 20.04 repositories), the attribute values were printed whenever the --verbose flag was given, even if the --data flag was not given. However, it seems that current version of h5ls prints attribute values only if --data flag is given. Was this change in behavior intentional?

In almost all of my use cases, the attributes are either scalars or small arrays, whereas the datasets can contain millions of values. In situations like these, it is usually not a good idea to print all the dataset values, but printing attribute values might provide useful information to the user. Currently, it is not possible to print attribute values without printing dataset values as well.

I think reverting back to the previous behavior (i.e., printing attribute values if verbose) would be an improvement. A probably better alternative would be to define a new flag, acting independently of the existing verbose and data flags, which specifies whether the attribute values should be printed.

[byrnHDF]

We could add an optional modifier to the --data argument such that:
--data -> both dataset and attribute data displayed
--data=1 -> dataset only data displayed
--data=2 ->attribute only data displayed