Resolve IS prefix in submodules (fixes #398)

[GabrielKS]

Pretty straightforward: I removed the IS prefix in submodules where it was not necessary and added IS aliases in each submodule for the cases where it is necessary. Right now there are no references to IS in IS.Simulation but I think it's probably a good practice to have the alias for the future anyway. Fixes #398.

[jd-lara]

Looks I left some of this code behind.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 78.28%. Comparing base (02e3688) to head (a353a06).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Optimization/optimization_problem_results.jl	0.00%	1 Missing ⚠️
src/Optimization/optimization_test_utils.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #399   +/-   ##
=======================================
  Coverage   78.28%   78.28%           
=======================================
  Files          71       71           
  Lines        5484     5484           
=======================================
  Hits         4293     4293           
  Misses       1191     1191           

Flag	Coverage Δ
unittests	78.28% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/Optimization/Optimization.jl	100.00% <ø> (ø)
src/Simulation/Simulation.jl	100.00% <ø> (ø)
src/Optimization/optimization_problem_results.jl	0.00% <0.00%> (ø)
src/Optimization/optimization_test_utils.jl	14.28% <0.00%> (ø)