`Group.__contains__` with nexus classes always returns True

[jl-wynen]

Given a Group group, the following always evaluates to True:

NXdata in group

And the same applies to all NX classes.

The reason is that we don't define Group.__contains__. So Python uses the fallback for mappings:

    def __contains__(self, key):
        try:
            self[key]
        except KeyError:
            return False
        else:
            return True

But group[NXdata] returns an empty dict if there is no NXdata in the group. So __contains__ is always True.

This is surprising behaviour. I think we have two options to fix it without breaking anything:

1. Actually check whether there are any children with the given class. This fixes the use case I have. But it creates an inconsistency between __contains__ and __getitem__.
2. Raise a TypeError when __contains__ is called with anything but a `str. This also creates an inconsistency. But I think this one is less severe.

Alternatively, we could change the behaviour of __getitem__ to raise when there is no child with the given class.

Or we leave it as. This keeps the mapping API consistent. But this can easily lead to errors downstream.

[jokasimr]

Alternatively, we could change the behaviour of __getitem__ to raise when there is no child with the given class.

I like this option best. It's least inconsistent and having __getitem__ raise when there's no matching value should be familiar to users.

Do we have a get function on Group? We could consider adding one to avoid having to do try: g[NXdata] except: etc. That would make the interface even more similar to other python mappings.