feat: skip histosys modifier creation for single-bin regions

[alexander-held]

There is no need to create histosys modifiers for single-bin regions, as the full effect is already absorbed into the normsys modifier created in that case.

* skip superfluous histosys modifier creation for single-bin regions

resolves #354

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (03f274f) 100.00% compared to head (dccbaa9) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #398   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           23        23           
  Lines         2065      2066    +1     
  Branches       326       327    +1     
=========================================
+ Hits          2065      2066    +1     

Impacted Files	Coverage Δ
src/cabinetry/workspace.py	100.00% <100.00%> (ø)

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