Allow manual specification of Tspan for models 1/2/3 in model.py

[vallis]

...as needed when analyzing a subset of pulsars with the CRN frequencies from the full array.

[codecov-commenter]

Codecov Report

Merging #172 (25d4069) into master (71783c1) will increase coverage by 0.20%.
The diff coverage is 96.15%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   37.13%   37.34%   +0.20%     
==========================================
  Files          20       20              
  Lines        3937     3950      +13     
==========================================
+ Hits         1462     1475      +13     
  Misses       2475     2475              

Impacted Files	Coverage Δ
enterprise_extensions/blocks.py	44.41% <50.00%> (ø)
enterprise_extensions/models.py	41.92% <100.00%> (+0.92%)	⬆️

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