Fix broken tensor executor test, allow tensor packets of size 1.

Antonio Sánchezrequested to merge
cantonios/eigen:vectorization into master

The cxx11_tensor_executor test assumed vectorization was always possible for the given types - though they may not be depending on the platform. Added a check via packet_traits<T>.

Also modified tensor ops to actually allow PacketSize == 1, such as for Packet1cd. The README previously said complex was known to be broken, but we've since added some fixes for that.

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