Fast LogSumExp over 4th dimension

[stevengj]

@jroon

I think then I’ll have to stick with my R version. Unless there is more improvement to be found here, the overhead of moving data from R to Julia which isn’t captured in my R benchmarks above that erases small benefits

In general, you won’t get much benefit from Julia from moving a single vectorized function to Julia. See also this post: Comparing Numba and Julia for a complex matrix computation - #3 by stevengj