Support for double vectors/matrices (with SIMD)?

[lasagnaphil]

Although this library is geared more towards graphics / game development than numeric computation, I think double-typed SIMD vectors will be very useful for implementing various physics simulations (for example, a lot of the fluid simulations tend to use doubles instead of floats because of numerical stability.)

One caveat is that we would have to write special code for SIMD for this feature, as the library only provides SIMD wrapper operations for floats. Maybe if I have the time I would try working on it...