Enable caching of compiled guvectorize functions

[tomwhite]

Fixes #363

$ time python -c "import sgkit"
real	0m5.481s
user	0m5.960s
sys	0m0.527s
$ time python -c "import sgkit"
real	0m1.982s
user	0m2.520s
sys	0m0.471s

BTW @aktech I noticed that the guvectorize metric functions don't have nopython=True - do you think we should add it? (I could do it in this PR.)

[jeromekelleher]

LGTM, minor query

[jeromekelleher]

Is there any good reason not to have cache=True for our use-case? Perhaps it's worth us having our own local guvectorize decorator that does this by default?

This is the sort of thing that's hard to test for and easy to forget, so maybe it's worth a little indirection.

[tomwhite]

Maybe? It's not hard to add and now we have the convention started it is likely to be propagated in our code.

[tomwhite]

Opened #371

[aktech]

BTW @aktech I noticed that the guvectorize metric functions don't have nopython=True - do you think we should add it? (I could do it in this PR.)

That will be a good addition, I think.

[aktech]

This is a really nice improvement, by the way. 👍

[codecov-io]

Codecov Report

Merging #364 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #364      +/-   ##
==========================================
+ Coverage   95.39%   95.41%   +0.01%     
==========================================
  Files          31       31              
  Lines        2108     2116       +8     
==========================================
+ Hits         2011     2019       +8     
  Misses         97       97              

Impacted Files	Coverage Δ
sgkit/distance/metrics.py	21.21% <ø> (ø)
sgkit/stats/aggregation.py	87.77% <ø> (ø)
sgkit/stats/popgen.py	67.74% <ø> (ø)
sgkit/variables.py	96.42% <ø> (ø)
sgkit/__init__.py	100.00% <100.00%> (ø)
sgkit/utils.py	100.00% <100.00%> (ø)

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