Expanded normalization modifiers

[ekourlit]

This PR expands the usage and notion of normalization modifiers. Now normalization modifiers can also come from weight or tree variations. For example, a weight-based systematic can define a variation from which a normalization effect is calculated.

This PR also allows the symmetrization of normalization systematics.

Changes are mainly affecting src/cabinetry/workspace.py where the workspace is created. src/cabinetry/configuration.py has also being changed so additional needed histograms are now build for normalization modifiers.

As a side note, it would be good to add a safety check during the creation of the configuration file in order to not allow different properties of Up and Down variations. For example, we might not want to allow:

  - Name: "my_systematic"
    Up:
      Weight: "<up-weight-string>"
    Down:
      Normalization: <up-factor>
    Type: "Normalization"