Fixed worldspace export flag behaviour to only bake xform at the top level prim

[csyshing]

This PR fixed tow problems:

• The worldspace flag for exporter to only bake xform at the top level prim, children prims xforms would be remained unchanged
• The logic to handle selection objects has also been modified to match mayaUsdPlugin that when selecting parent node and one of the children, unselected siblings would be excluded, for example:

- parent            * selected
   - child_A       * selected
   - child_B

child _B object would be excluded when exporting.

[csyshing]

Jenkins test failed due to this one:

    ======================================================================
    FAIL: test_anonymousLayerSerialisation (testLayerManager.TestLayerManagerSerialisation)
    Tests multiple anonymous sublayers can be saved and restored.
    ----------------------------------------------------------------------
    Traceback (most recent call last):
      File "/home/S/jenkins/workspace/ecg-mayausd-branch-preflight-2020-pxrusd-mid3-interactive-linux/ecg-maya-usd/maya-usd/plugin/al/plugin/AL_USDMayaTestPlugin/py/testLayerManager.py", line 262, in test_anonymousLayerSerialisation
        self.assertIsNotNone(sublayer02)
    AssertionError: unexpectedly None
    
    ----------------------------------------------------------------------

Strangely this is the same reason for other PRs e.g. #1924 .

The unit test above should have been fixed (and have been merged) in c65e1d7

Any idea?

Update: did a fast-forward and the unit test should pass fine now.

[seando-adsk]

@csyshing Is there someone who can code review these changes? Once completed I will merge this PR.

[csyshing]

@seando-adsk , this PR has been reviewed internally in Animal Logic; on external, usually @kxl-adsk would do the review?

[seando-adsk]

@csyshing Okay. Krystian is off this week. Are you okay waiting until next week? If not we can find someone else to review.

[csyshing]

No problem @seando-adsk , it's not urgent to merge this PR.