IECoreUSD : Import some core USD modules before _IECoreUSD

Without this, at IE, we get the warning message:
SystemError: initialization of _tf raised unreported exception

I don't know whether this is a reasonable solution, but I think I'll just PR it for discussion.

Author: danieldresser-ie

contrib/IECoreUSD/python/IECoreUSD/__init__.py

@@ -32,6 +32,14 @@
 #
 ##########################################################################
 
+# In the ImageEngine env, if _IECoreUSD is imported before these pxr modules,
+# we get a weird exception when we later load the pxr modules. We're not
+# 100% sure why this is happening, but importing them first isn't
+# unreasonable.
+__import__( "pxr.Kind" )
+__import__( "pxr.Vt" )
+__import__( "pxr.Sdf" )
+
 from ._IECoreUSD import *
 
 __import__( "IECore" ).loadConfig( "CORTEX_STARTUP_PATHS", subdirectory = "IECoreUSD" )