Make storage more space efficient

[cschwan]

This branch introduces a new data structure intended to store the (Q^2, x1, x2) grids in a more space-efficient way.

[codecov]

Codecov Report

Merging #48 (4663ea1) into master (0dd5689) will increase coverage by 3.43%.
The diff coverage is 95.03%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   87.46%   90.89%   +3.43%     
==========================================
  Files           7        8       +1     
  Lines         654      934     +280     
==========================================
+ Hits          572      849     +277     
- Misses         82       85       +3     

Impacted Files	Coverage Δ
pineappl/tests/drell_yan_lo.rs	100.00% <ø> (ø)
pineappl/src/grid.rs	81.45% <88.88%> (+0.22%)	⬆️
pineappl/src/lagrange_subgrid.rs	93.70% <94.33%> (+7.70%)	⬆️
pineappl/src/sparse_array3.rs	95.80% <95.80%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0dd5689...4663ea1. Read the comment docs.