ADD: functions and capabilitites that automatically initialize the boundary power flow variables.

[juanjospina]

• Added functions and capabilitites that automatically initialize the boundary power flow variables (i.e., pbound_fr, pbound_to, qbound_fr, and qbound_to).
• p/qbound_fr variables are initialized based on the data parsed from the transmission system data.
• p/qbound_to variables are initialized by adding all the loads in the respective distribution systems and dividing them by 3 (assuming the initial state of the distribution system is balanced).
• Refactored multiple functions in ref.jl to avoid unnecessary loops. These unnecessary loops were making the refs processes unnecessary long.

[codecov]

Codecov Report

Merging #15 (c7cc5f1) into main (bdece4d) will increase coverage by 0.14%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #15      +/-   ##
==========================================
+ Coverage   79.61%   79.75%   +0.14%     
==========================================
  Files          28       28              
  Lines        4007     4036      +29     
==========================================
+ Hits         3190     3219      +29     
  Misses        817      817              

Impacted Files	Coverage Δ
src/core/variable.jl	100.00% <ø> (ø)
src/core/ref.jl	100.00% <100.00%> (ø)

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