Add important documentation to the structs in src/system_structure.jl

[1-Bart-1]

• Add docstrings for the SystemStructure components constructors
• Add docstrings for the SymbolicAWEModel constructors

[codecov]

Codecov Report

Attention: Patch coverage is 95.08197% with 3 lines in your changes missing coverage. Please review.

Project coverage is 87.18%. Comparing base (2100066) to head (947ac8a).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
src/system_structure.jl	94.82%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #229      +/-   ##
==========================================
+ Coverage   86.86%   87.18%   +0.31%     
==========================================
  Files           9        9              
  Lines        2093     2114      +21     
==========================================
+ Hits         1818     1843      +25     
+ Misses        275      271       -4     

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