USD PrimitiveAlgo : Fix bugs handling normals

[johnhaddon]

• primvars:normals takes precedence over UsdGeomPointBased normals when reading.
• Writing always creates primvars:normals, because it can round-trip indexed normals while UsdGeomPointBased normals can not.

Note that I haven't updated the code for loading skinned normals. I don't have test data for this, and USD doesn't expose skinning of facevarying or indexed primvars in UsdSkelSkinningQuery, so fixing that is a bigger task.

[danieldresser-ie]

LGTM.

Would be nice to fix the test tolerance that flagged this check as failed, I've noticed this one a few times on my own PRs as well:

FAIL: testCancel (AlembicSceneTest.AlembicSceneTest)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/__w/cortex/cortex/contrib/IECoreAlembic/test/IECoreAlembic/AlembicSceneTest.py", line 1937, in testCancel
    self.assertLess( time.time() - startTime, 0.06 )
AssertionError: 0.06280231475830078 not less than 0.06```

[johnhaddon]

Would be nice to fix the test tolerance that flagged this check as failed

PR welcome :) I think we can just skip that test for debug builds - we can't expect time-sensitive stuff to behave there.