Integrated nuclear fits

[Radonirinaunimi]

A re-implementation of the integrated nuclear-PDF fits based on the refactors.

Remaining to do:

• [ ] Improve and re-organize the writer and vpinterface modules
• [ ] Accommodate the computation of integrability and arc-lengths to properly account for $A \neq 1$
• [ ] Properly write the predictions onto disk and modify the way evolution is applied accordingly
• [ ] Fix broken tests
• [ ] Extend the report to include nuclear predictions

Checks:

• [x] Standard fits in which only proton-like data are included (report)
• [ ] ...