Revert "Change Symbols to Strings for input data"

[abelsiqueira]

Reverts #711 as discussed. Let's see if the speedup is equivalent.

[github-actions]

Benchmark Results

	434fd43...	e1c763c...	434fd43.../e1c763c0116d51...
energy_problem/create_model	0.13 ± 0.0071 h	0.127 ± 0.0079 h	1.02
energy_problem/input_and_constructor	15.3 ± 0.13 s	13.8 ± 0.1 s	1.11
time_to_load	2.9 ± 0.019 s	3.01 ± 0.056 s	0.963

	434fd43...	e1c763c...	434fd43.../e1c763c0116d51...
energy_problem/create_model	0.794 G allocs: 32.9 GB	0.794 G allocs: 32.8 GB	1
energy_problem/input_and_constructor	6.39 M allocs: 0.661 GB	6.39 M allocs: 0.674 GB	0.982
time_to_load	0.153 k allocs: 14.5 kB	0.153 k allocs: 14.5 kB	1

Benchmark Plots

A plot of the benchmark results have been uploaded as an artifact to the workflow run for this PR.
Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (434fd43) to head (e1c763c).

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #720   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           16        16           
  Lines          768       770    +2     
=========================================
+ Hits           768       770    +2     

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