Resampling of different flavours per sample

As discussed in !686 (comment 6286667), it would be nice to allow for resampling with different flavours per sample (i.e. different categories per type in umami-config-speak).

This could work by the introduction of a "region" (e.g. in pt) alongside the already specified sample and flavour. Then all the different flavours in the different samples in a given region are resampled. For standard hybrid sample we would specify something like

ttbar_lowpt:
  region: low_pt
  sample: ttbar
  flavours: [b, c, l]

zprime_highpt:
  region: low_pt
  sample: zprime
  flavours: [b, c, l]

for Xbb, we would have

Hbb:
  region: inclusive
  sample: vhbb
  flavours: Hbb

Hbb:
  region: inclusive
  sample: vhcc
  flavours: Hcc

...

This would be more flexible, and allow us to also extend the hybrid sample more easily (for example adding graviton alongside zprime in the high-pt region.