Support for zerocorr in the bootstrap

[palday]

• expand the type definition for the original fit (which I view as bugfix-esque, so patch)
• added a Base.length method (technically new functionality so minor release)

Closes #560

[codecov]

Codecov Report

Merging #561 (03011a0) into main (a06440a) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #561   +/-   ##
=======================================
  Coverage   96.14%   96.14%           
=======================================
  Files          27       27           
  Lines        2489     2490    +1     
=======================================
+ Hits         2393     2394    +1     
  Misses         96       96           

Impacted Files	Coverage Δ
src/bootstrap.jl	97.85% <100.00%> (+0.01%)	⬆️

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