Pruning modifiers per sample and channel

[alexander-held]

Description

Modifier pruning currently removes a modifier completely from a model. It would be convenient to be able to remove modifier effects per sample or channel via the pruning API. This can be useful for example when a systematic has very different effects across phase space or between samples, and can reduce the amount of interpolations/extrapolations that need to be calculated.

Pruning is useful to speed up fits without significant impact on the results (if done correctly). See these slides (ATLAS internal) for a possible approach (@kratsg).

Is your feature request related to a problem? Please describe.

pruning currently is all-or-nothing, for more customization an external implementation is needed

Describe the solution you'd like

per-sample and per-channel modifier pruning

Describe alternatives you've considered

external implementation

Relevant Issues and Pull Requests

nothing immediately relevant, but some related issues:

• Pruning of gammas #662
• Fast, Inaccurate Mode #1056
• Removing pruned parameters from measurement #1142
• Pruning bins per channel #1143
• Pruning part of a shared modifier only works in specific order #1144

Additional context

none