Fix 86

[zakv]

This PR resolves #86 by making Run.get_units() only return units for the group specified by its optional group argument. If it is left at its default value of None, then the units for all globals in all groups are returned as before. Also, a docstring was added for that method.

[philipstarkey]

Wondering if the following is a cleaner implementation (the append_units function seems overly complex for what it does).

    def get_units(self, group=None):
        units = {}
        path = 'globals'
        if group is not None:
            path += f'/{group}'
        with h5py.File(self.h5_path, 'r') as h5_file:
            h5_file[path].visititems(
                lambda n, o: units.update(dict(o.attrs) if 'units' in n else {})
            )
        return units

(This has not been tested. I'm not even sure if that dict above is necessary either)

There are two main changes in this:

1. Dynamically constructing path rather than using if/else
2. Ditching the function for a lambda.

The first change is probably cleaner, but second might not be worth it. Could instead use:

        def append_units(name, obj):
            if 'units' in name:
                units.update(dict(obj.attrs))

(I think)

EDIT: Oh, and these examples are missing the try/except to catch invalid group name!

[zakv]

Yeah you're right that method could be simplified a bit. I still went with append_units() over the lambda function just because I thought it made the code a bit easier to read/understand, though maybe a bit less pythonic.