power spectrum module optimization and parallelization

[lgarrison]

Some optimization, parallelization, and Numba-fication of various parts of the power spectrum module. Benchmark scripts used to produce the timings in the ZCV paper are included.

[lgarrison]

@boryanah Here are the changes we talked about. It's about half optimization and half refactoring. I tried to rename some things that were confusing to me, but let me know if I misunderstood anything. I also tried to cut down on the number of arguments and return values in some places. For example, I changed calc_power() to return an Astropy Table; let me know if you think that makes sense.

[boryanah]

That looks great to me! The optimizations make sense (I am sorry I didn't implement the normalization one myself). The astropy table for the power spectrum with the meta data is great, and I think it's a good solution for outputting a single object that contains some useful information about the simulation. I think it makes sense why you got rid of some of the del X; gc.collect() (they probably weren't doing anything as the variables were passed externally rather than locally defined). I also like the variable and function name changes, and I think they make more sense.

[lgarrison]

Yeah, that's exactly right about the gc.collect(). The other reason was that garbage collection was making the timings really noisy; there might be one or two that could go back in, but for the most part I think they weren't doing anything. And if we really wanted to save memory, there are other ways: using an in-place FFT (pyfftw supports this, not sure if scipy.fft does), and adding on-the-fly offsets for the interlacing calculation (right now it makes a whole copy of the input data).