Add subcommand `export` to convert PineAPPL grids

[cschwan]

TODO list:

• [x] static scale support; added in commit edf095450f79edf4090b13377514c2d4cb0f074a
• [x] more robust grid value detection; added in commit 51bb4dae799475f8b152f6c1d956b41500d7afe2 and f88f980a9a452b759683b3ecf6e7a7045c90fea8
• [x] add Q2, x1 and x2 minimum- and maximum value detection
• [x] fix photon contributions; added in commit 937b1a13b15ea9b6e95ce6af257b3845d56cb605
• [x] add integration tests; added in commit 37f8290ad94907e3e75b18ee22c492deb4b856af
• [x] add DIS support; added in commit ce1912bea44feb5e34c11c57ff4046ad0ffa2a94