ENH: enabling 64-bit pixel IDS

Change-Id: I4b42b9a53d2618d1af84ed4a7bb2e2833f7818a5
ENH: updating CastImageFilterTest for int64 images

Change-Id: Ia9b4f5456b1097e3d028f8db33f8a4f67a7990d5
Adding numpy test for int64 images

Change-Id: I37f0c4e1bc53461ecfa6e01f1b005c5b1b125ea1
Adding Get/Set pixel tests for int64 images

Change-Id: I94d1f35b1a685d13f63e7adb4d0984c7109db23f
Adding support to import int64 images

Change-Id: I39e098e63f565e0bc0ed5ae662bbb2e245e16c48
Merge topic 'SIMPLEITK-397_AddInt64Pixels' into next

4fc74e2 Adding support to import int64 images
ecd3163 Adding Get/Set pixel tests for int64 images
7bb92ce Adding numpy test for int64 images
ce1d8fb ENH: updating CastImageFilterTest for int64 images
51baa49 ENH: enabling 64-bit pixel IDS