Decrease grid file size

[cschwan]

The grids that are currently produced are rather large (CMSDY2D11 is 800 MB large), but this can be optimized using several strategies:

• many of out grids have basically a static scale, so that we should be able to remove the interpolation in Q2,
• inside the non-zero Q2 grids many bins are probably empty, which can be optimized when writing to disk using a simple compression algorithm.

[cschwan]

A simple strategy is to compress the grids. I've tried the following compression algorithms:

0. without compression the file is 894 MB according to du --apparent-size,
1. lzop --fast: 139 MB, lzop --best: 135 MB,
2. lz4 --fast: 137 MB, lz4 --best: 133 MB,
3. gzip --fast: 133 MB, gzip --best: 130 MB,
4. bzip2 --fast: 127 MB, bzip2 --best: 126 MB,
5. 7z a -t7z -mx=1: 127 MB, 7z a -t7z -mx=9: 125 MB,
6. lzip --fast: 126 MB, lzip --best: 124 MB,
7. xz --fast: 125 MB, xz --best: 125 MB.

A disadvantage is that compressing the grids (which we can do once) adds a decompression penalty every time we read it. Therefore the following timings are relevant (each using a compressed grid with the fastest compression algorithm on dom):

1. lzop -d: real 0m0.806s, user 0m0.413s, sys 0m0.393s,
2. lz4 -d: real 0m0.640s, user 0m0.203s, sys 0m0.436s,
3. gzip -d: real 0m2.776s, user 0m2.335s, sys 0m0.440s,
4. bzip2 -d: real 0m7.026s, user 0m6.550s, sys 0m0.473s,
5. 7z e: not available on dom,
6. lzip -d: not available on dom,
7. xz: real 0m7.550s, user 0m7.050s, sys 0m0.490s.

Here also the corresponding numbers with the best compression:

1. lzop -d: real 0m1.136s, user 0m0.759s, sys 0m0.369s,
2. lz4 -d: real 0m0.629s, user 0m0.203s, sys 0m0.425s,
3. gzip -d: real 0m3.335s, user 0m2.947s, sys 0m0.387s,
4. bzip2 -d: real 0m8.619s, user 0m8.120s, sys 0m0.493s,
5. 7z e: not available on dom,
6. lzip -d: not available on dom,
7. xz: real 0m7.571s, user 0m7.061s, sys 0m0.494s.

[cschwan]

Commit 1f4df99 implements reading from an LZ4 compressed stream, using the lz-fear crate. The decompression penalty is roughly 0.5 seconds for the usecase shown above, which is what the tests predicted. If we decide the that the grids are too large, we can simply compress them, if not we leave them. PineAPPL can read both now.

[cschwan]

PR #48 implements another file-size optimisation, and Issue #45 lists more possibilities.