feat: support SamplePath overrides for histogram inputs

[alexander-held]

As discussed in #435 and originally via #433, SamplePath overrides are needed to provide additional flexibility for histogram inputs. This adds support for such overrides. For easier debugging, a new DEBUG level message is also added to report the location of histogram and ntuple inputs.

* support SamplePath overrides when using histogram inputs
* update documentation for histogram inputs
* add debug-level log message with paths to ntuple and histogram inputs

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2b7c091) 100.00% compared to head (7ba0e62) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #437   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           22        22           
  Lines         2071      2082   +11     
  Branches       338       340    +2     
=========================================
+ Hits          2071      2082   +11     

Files Changed	Coverage Δ
src/cabinetry/templates/builder.py	100.00% <ø> (ø)
src/cabinetry/contrib/histogram_creator.py	100.00% <100.00%> (ø)
src/cabinetry/contrib/histogram_reader.py	100.00% <100.00%> (ø)
src/cabinetry/templates/collector.py	100.00% <100.00%> (ø)

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