Add basic support for PST in DC OPF models

[ccoffrin]

closes #543

[codecov]

Codecov Report

Merging #875 (06816e5) into master (eb21c13) will increase coverage by 0.04%.
Report is 1 commits behind head on master.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #875      +/-   ##
==========================================
+ Coverage   93.81%   93.86%   +0.04%     
==========================================
  Files          43       43              
  Lines        9893     9973      +80     
==========================================
+ Hits         9281     9361      +80     
  Misses        612      612              

Files Changed	Coverage Δ
src/core/constraint_template.jl	96.17% <100.00%> (+0.15%)	⬆️
src/form/acp.jl	93.43% <100.00%> (+0.51%)	⬆️
src/form/dcp.jl	97.33% <100.00%> (+0.15%)	⬆️
src/prob/test.jl	94.55% <100.00%> (+0.41%)	⬆️

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[ccoffrin]

Adding a new formulation _solve_opf_pst which only addresses pst optimization instead of both oltc and pst, _solve_opf_oltc_pst. This enables a clean implementation of a DCP and ACP formulations without having to consider the impact on oltc on the DCP approximation.