[eigen] New(?) way to make using SIMD easier

[Mark Borgerding <mark@xxxxxxxxxxxxxx>]

Just an idea:
What if the user could write code like:

   VectorOperator( std::plus<SomeDataType>() , dstPtr , srcPtr1, 
srcPtr2 , num );

which would use a SIMD-optimized call if one exists, and use a generic 
algorithm otherwise.

Examples:
1. a functor std::plus<float>  and float pointers:    there could be a 
specialization that uses _mm_add_ps (on x86) to add 4 floats at a time
2. a functor std::multiplies<double> and double pointers:    the 
specialization would be to use _mm_mul_pd to add 2 doubles at a time

It might even be used to detect special conditions.  e.g. If CUDA 
processing is enabled and the source pointers are device memory  It 
performs all calculations on the device.  Then brings the result back to 
the host only if the destination resides in host memory.

Does this offer any advantages in development speed/clarity  or in 
flexibility over Eigen's current method of using SIMD?  I admit I am 
fairly ignorant of the current state of Eigen SIMD.