Extend support for overrides for path construction with histogram and ntuple inputs

[alexander-held]

433 revealed that overrides for path construction with histogram inputs only covered {VariationPath} and not the region- and sample-specific options. At least sample-specific overrides seem like they could be frequently needed.

Investigate which additional options need to be supported for both ntuple and histogram inputs. Relevant documentation: https://cabinetry.readthedocs.io/en/latest/core.html

A VariationPath setting may be needed for ntuples in case of multi-tree ntuples with one tree per systematic.

[alexander-held]

A few more thoughts on this:

general:

• use cases for RegionPath overrides are unclear to me

ntuple inputs:

• RegionPath / SamplePath overrides are already supported
• the Tree setting might fully capture the VariationPath functionality already (?)
  • does it make sense to harmonize these settings?

histogram inputs:

• VariationPath is supported, but since it is set per-systematic it cannot capture use cases where both file name and histogram name may change (e.g. detector systematics all in the same file, two-point modeling systematics in different files)
• SamplePath is not yet supported but needed for use cases like #433 -> added via #437
• RegionPath is also not supported at the moment, use case not clear