Add type assertions in add_expression and attach_expression functions

[abelsiqueira]

Add type assertions in add_expression and attach_expression functions

This should help make it a bit faster by making all three functions more type stable. All three functions are similarly typed now.

[github-actions]

Benchmark Results

	e8e59d5...	5d865ad...	e8e59d5... / 5d865ad...
energy_problem/create_model	35.3 ± 2 s	33 ± 1.9 s	1.07
energy_problem/input_and_constructor	27.4 ± 0.18 s	27.7 ± 0.11 s	0.989
time_to_load	2.61 ± 0.023 s	2.63 ± 0.016 s	0.992

	e8e59d5...	5d865ad...	e8e59d5... / 5d865ad...
energy_problem/create_model	0.243 G allocs: 12.1 GB	0.204 G allocs: 11.4 GB	1.06
energy_problem/input_and_constructor	26.8 M allocs: 0.952 GB	26.8 M allocs: 0.952 GB	1
time_to_load	0.159 k allocs: 11.2 kB	0.159 k allocs: 11.2 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 97.86%. Comparing base (e8e59d5) to head (5d865ad).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1055   +/-   ##
=======================================
  Coverage   97.86%   97.86%           
=======================================
  Files          29       29           
  Lines         985      985           
=======================================
  Hits          964      964           
  Misses         21       21           

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[clizbe]

Looks good to me!