[WIP] Separable GPs

[willtebbutt]

The intention of this PR is to develop an implementation of separable GPs in Stheno, hoperfully addressing #6 by the time that we're done.

@MichielStock I'll carry on working on this, but let me know what you think. I've just implemented it for problems involving two input dimensions, and I'm hoping that we can nest stuff in a similar manner to Kronecker.jl and automatically get arbitrary numbers of inputs...

[willtebbutt]

Currently gives reasonably performance:

using Stheno, BenchmarkTools
using Stheno: GPC, RectilinearGrid, Separable

f = GP(Separable(eq(), matern32()), GPC());

xl = range(-3.0, 3.0; length=100);
xr = range(-2.5, 2.5; length=100);
x = RectilinearGrid(xl, xr);

y = rand(f(x, 0.1));
logpdf(f(x, 0.1), y)

fx = f(x, 0.1);
@benchmark rand($fx)
y = rand(fx);

@benchmark logpdf($fx, $y)

produces (rand)

BenchmarkTools.Trial: 
  memory estimate:  1.38 MiB
  allocs estimate:  97
  --------------
  minimum time:     7.600 ms (0.00% GC)
  median time:      7.817 ms (0.00% GC)
  mean time:        8.392 ms (0.46% GC)
  maximum time:     32.777 ms (0.00% GC)
  --------------
  samples:          596
  evals/sample:     1

and (logpdf)

BenchmarkTools.Trial: 
  memory estimate:  1.15 MiB
  allocs estimate:  87
  --------------
  minimum time:     6.062 ms (0.00% GC)
  median time:      6.425 ms (0.00% GC)
  mean time:        6.988 ms (0.49% GC)
  maximum time:     32.850 ms (0.00% GC)
  --------------
  samples:          715
  evals/sample:     1

on my machine, which seems about right.

[codecov]

Codecov Report

Merging #47 into master will decrease coverage by 2.45%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
- Coverage   85.93%   83.48%   -2.46%     
==========================================
  Files          24       25       +1     
  Lines         647      666      +19     
==========================================
  Hits          556      556              
- Misses         91      110      +19

Impacted Files	Coverage Δ
src/Stheno.jl	100% <ø> (ø)	⬆️
src/extras/separable.jl	0% <0%> (ø)

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