Fix DataFrame creation in 08_US_system.jl

[josephmckinsey]

08_US_system.jl uses a very deprecated method to create a DataFrame. In DataFrames.jl 1.0, these methods were removed (JuliaData/DataFrames.jl#2679).

[codecov]

Codecov Report

Merging #44 (8fbacaa) into master (6872368) will decrease coverage by 0.78%.
The diff coverage is n/a.

❗ Current head 8fbacaa differs from pull request most recent head 0157441. Consider uploading reports for the commit 0157441 to get more accurate results

@@            Coverage Diff            @@
##           master     #44      +/-   ##
=========================================
- Coverage    8.00%   7.21%   -0.79%     
=========================================
  Files           3       3              
  Lines         100      97       -3     
=========================================
- Hits            8       7       -1     
+ Misses         92      90       -2     

Flag	Coverage Δ
unittests	7.21% <ø> (-0.79%)	⬇️

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Impacted Files	Coverage Δ
src/utils.jl	21.21% <0.00%> (-2.32%)	⬇️
src/launch.jl	0.00% <0.00%> (ø)

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[claytonpbarrows]

Thanks!