Add PWL average systems for PSI testing

[GabrielKS]

To support Sienna-Platform/PowerSimulations.jl#1446, we need some test systems with PiecewiseAverageCurves in them. Here I create such by using our built-in conversions to transform ValueCurves in existing incremental systems — thus we have that c_pwl_average_cost_test should represent the same system as c_pwl_incremental_cost_test and c_pwl_average_fuel_test as c_pwl_incremental_fuel_test, except with PiecewiseAverageCurves instead of PiecewiseIncrementalCurves.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.66%. Comparing base (77d1112) to head (20964dc).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #181      +/-   ##
==========================================
+ Coverage   92.62%   92.66%   +0.04%     
==========================================
  Files          18       18              
  Lines        4458     4474      +16     
==========================================
+ Hits         4129     4146      +17     
+ Misses        329      328       -1     

Flag	Coverage Δ
unittests	92.66% <100.00%> (+0.04%)	⬆️

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Files with missing lines	Coverage Δ
src/library/psitest_library.jl	99.26% <100.00%> (+<0.01%)	⬆️

... and 1 file with indirect coverage changes

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