Always return pointer object on indexing

[pmrv]

Currently indexing to a non exiting group shows to distinct behaviors. If the file exists a new pointer object is returned and writing to it will create the group. If the file does not exist None is returned. This patch makes both cases return a pointer object which creates the underlying file/group once written to it. Imo this is more convenient for downstream users because otherwise they need to maintain separate code paths for creating new groups in existing and non existing files, since there are no methods to force creation of an empty group (what would be create_group in pyiron_base speak).

[jan-janssen]

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FAIL: test_empty_file_root (test_pointer.TestPointer)
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Traceback (most recent call last):
  File "/home/runner/work/h5io_browser/h5io_browser/tests/test_pointer.py", line 42, in test_empty_file_root
    self.assertIsNone(p["test"])
AssertionError: <h5io_browser.pointer.Pointer object at 0x7fb8e3287a30> is not None

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