Implement v1 file format [WIP]

[t7phy]

TODO:

• Generalize Channel to support arbitrarily many PIDs. This is implemented with commits f32902a, 2417c92 and 03f2dc6.
• The structure Order should support a fragmentation scale. Started with commits 5e281ae and 02e48e9.
• The CLI should support varying this new fragmentation scale. We need the following point variations: 1 (no scale variation), 3 (vary all scales with the same factor), 7 and 9 (assuming that there's no fragmentation scale), and 17 and 27 (assuming there's a fragmentation scale)
• Extend the evolution code to take care of (at least) one, two and three convolutions with possibly one, two and three different EKOs
• Finally, we need new subgrid types that support possibly arbitrarily many scales and momentum fractions and which support filling or are optimized for space. We should use the PackedArray struct that we wrote some time ago.
• To fix the tests, we need import v0 PineAPPL grids and convert the subgrids into the new subgrid types.

[cschwan]

I believe the list shown in #118 is probably a bit too ambitious for this pull request. Realistically, we should implement all the items listed in #172.