Fix bug in calc_theta_delta_bounds

[ccoffrin]

closes #907

[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 94.19%. Comparing base (8a1dc36) to head (80537f9).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #908      +/-   ##
==========================================
- Coverage   94.22%   94.19%   -0.03%     
==========================================
  Files          43       43              
  Lines        9612     9618       +6     
==========================================
+ Hits         9057     9060       +3     
- Misses        555      558       +3     

Files	Coverage Δ
src/core/data.jl	91.98% <71.42%> (-0.10%)	⬇️

... and 1 file with indirect coverage changes

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