Add fast_deepcopy_system function

[GabrielKS]

This feature supports the PSY implementation of NREL-Sienna/PowerSystems.jl#1231 (i.e., NREL-Sienna/PowerSystems.jl#1234) by adding a function fast_deepcopy_system that makes a deepcopy of a SystemData more quickly by skipping the copying of time series and/or supplemental attributes.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.07%. Comparing base (180ab67) to head (47c4ff6).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #421      +/-   ##
==========================================
+ Coverage   77.75%   78.07%   +0.31%     
==========================================
  Files          69       69              
  Lines        5481     5506      +25     
==========================================
+ Hits         4262     4299      +37     
+ Misses       1219     1207      -12     

Flag	Coverage Δ
unittests	78.07% <100.00%> (+0.31%)	⬆️

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

Files with missing lines	Coverage Δ
src/system_data.jl	91.80% <100.00%> (+0.52%)	⬆️

... and 1 file with indirect coverage changes

[daniel-thom]

LGTM