More NFlavours: support NFlavour=3,4,5,6 and new parameter isFFNS
Added support for NFlavour=3,4,5,6 in QCDNUM and APFEL (but not APFEL++) evolutions.
Before only NFlavour=3 and NFlavour=5 were available (moreover NFlavour=5 was actually NFlavour=6).
New parameter isFFNS (in constants.yaml) controls whether it is FFNS or VFNS:
- isFFNS = 0 (default, backward compatible with existing constants.yaml): VFNS with number of flavours changing from 3 to NFlavour
- isFFNS = 1: FFNS with number of flavours = NFlavour at any scale
Thus now the following scenarios are available:
- isFFNS = 0, NFlavour = 6: VFNS with from 3 to 6 flavours
- isFFNS = 1, NFlavour = 6: 6 flavours at any scale (thus in practice at the starting scale Q_0^2 as well)
- isFFNS = 0, NFlavour = 5: VFNS with from 3 to 5 flavours (in practice equivalent to isFFNS = 0, NFlavour = 6 which was default in the past)
- isFFNS = 1, NFlavour = 5: 5 flavours at any scale
- isFFNS = 0, NFlavour = 4: VFNS with from 3 to 4 flavours
- isFFNS = 1, NFlavour = 4: 4 flavours at any scale
- isFFNS = 0, NFlavour = 3: 3 flavours at any scale (equivalent to VFNS with from 3 to 3 flavours)
- isFFNS = 1, NFlavour = 3: 3 flavours at any scale (thus isFFNS = 0, NFlavour = 3 and isFFNS = 1, NFlavour = 3 are equivalent, and this is equivalent to the old NFlavour = 3 setting used e.g. in the FFABM test)
@aglazov could you please check the modification to the QCDNUM evolution, in particular to qcdnumDef? It was needed to have FFNS with 4, 5 or 6 flavours and avoid Input quark densities not linearly independent error from QCDNUM, but I do not have much experience with QCDNUM routines and do not know if what I did was optimal?
This MR was initially triggered by the request from Sara to have FFNS with 4 flavours (apparently, a VFNS with 3 to 4 flavours was requested), even though in the old xFitter: https://groups.google.com/forum/#!msg/xfitter-users/tcuwZNDEgpc/wm0Eagr1AAAJ
Also four tests VFNS4-QCDNUM, FFNS4-QCDNUM, VFNS4-APFEL, FFNS4-APFEL are added (QCDNUM and APFEL produce identical evolved PDFs).
