Optimise memory usage: GPU array storage and other small changes

[jakewilliami]

#24205 in julia lang https://github.com/JuliaLang/julia/issues/

[jakewilliami]

Namely after "smart" feature selection

[jakewilliami]

Done in commit e911698.

[jakewilliami]

#25 addresses this broader issue of memory efficiency

[jakewilliami]

Commit 61b0382 addresses this

[jakewilliami]

Points to consider and possibly implement:

• GPU processing
  • Use ArrayFire.jl's load_image
  • Use CUDA.jl
• Use 4 x ones or -ones using Int8 (ask @dmipeck)
• pmap
• Mmap
• Use SharedArrays

[jakewilliami]

Commit 70d7d74 addresses this

[jakewilliami]

Some overview thus-far.

These benchmarking results are from tests since we changed the algorithm to run sequentially.

Commit	Benchmark Time of Tests (seconds)	% Time Improvement Since Previous Listed Commit	Number of Allocations	Memory Allocation	% Memory Improvement Since Previous Listed Commit
a4689195	30.689	— a	371318354	6.38 GiB	— a
da9c833e	7.768	74.69	104464112	2.51 GiB	60.66
b3aec6b8	5.025	35.31	28589987	713.89 MiB	71.56
3e9be4ad	5.242	-4.32 b	46688538	990.05 MiB	-38.68 b

^a I did not benchmark prior to this, though it probably wouldn't be hard to checkout and rewrite some tests with an older version.
^b In this commit, I had to change the output of the get_vote function from Int8 to Float64 for correctness. As a result, this is why we have a decrease in benchmarks since the previous listed commit.

---

NB—: time improvement since last commit can be calculated very easily:

julia> improvement(a, b) = ((a - b) / a) * 100
improvement (generic function with 1 method)

julia> improvement(30.689, 7.768)
74.68799895728111

That is to say, there was a 74.7% improvement between times 30.689 s and 7.768 s.