Refactoring of ExaPF

[frapac]

• simplify drastically the code (would help future maintenance)
• all functions are now implemented in src/Polar/functions.jl with the nonlinear basis introduced in https://arxiv.org/abs/1906.10652. This allows to streamline the evaluations of the callbacks with sparse linear algebra (mostly SpMV operations).
• remove old code

A tutorial explaining the new implementation is coming.

[frapac]

This PR addresses #199 #85 #28

[codecov-commenter]

Codecov Report

Merging #214 (88de7da) into develop (7bb6b4d) will decrease coverage by 7.92%.
The diff coverage is 90.26%.

@@             Coverage Diff             @@
##           develop     #214      +/-   ##
===========================================
- Coverage    78.60%   70.67%   -7.93%     
===========================================
  Files           37       27      -10     
  Lines         4039     2916    -1123     
===========================================
- Hits          3175     2061    -1114     
+ Misses         864      855       -9     

Impacted Files	Coverage Δ
src/PowerSystem/PowerSystem.jl	97.91% <ø> (-0.30%)	⬇️
src/models.jl	100.00% <ø> (ø)
src/cuda_wrapper.jl	54.76% <54.76%> (ø)
src/autodiff.jl	68.96% <68.42%> (-7.46%)	⬇️
src/Polar/first_order.jl	87.03% <87.03%> (ø)
src/Polar/functions.jl	90.25% <90.25%> (ø)
src/LinearSolvers/LinearSolvers.jl	64.28% <100.00%> (-9.05%)	⬇️
src/Polar/legacy.jl	100.00% <100.00%> (ø)
src/Polar/newton.jl	100.00% <100.00%> (ø)
src/Polar/polar.jl	100.00% <100.00%> (ø)
... and 8 more

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