[eigen] vectorization and ABI

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I think again that we need to make sure that enabling/disabling vectorization 
does not affect the ABI. The use case is for apps who want to make a runtime 
switch for vectorization. They would compile the same file a.cpp with and 
without -msse2. If a.cpp contains functions that take (refs to) Eigen 
matrices as argument, this requires the type of Eigen matrices to be 
independent of whether -msse2 is used. This is currently not the case since 
PacketAccessBit is set to 0 unless vectorization is enabled. So I think we 
must change that. I understand that setting PacketAccessBit to 0 was a nice 
convenience to quickly ensure that vectorized paths are never used without 
vectorization support, but we can do without... so I'm for setting 
PacketAccessBit regardless of the vectorization switch; and we could have 
another constant ActualPacketAccess that is the same thing that is currently 
called PacketAccessBit. What do you think? I know we discussed that before 
but I don't remember clearly the outcome.



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