FIX: Bugs in center tap transformer model

[keatsig]

This PR fixes the following bugs in center-tapped transformer model:

1. Corrected polarity of one of the transformers on secondary side connected between terminals [4,2] to introduce the 180 degree phase shift at the node with terminal [2].
2. Fixed the terminal numbering of virtual nodes which were always starting with 1 even if transformer was not connected to terminal [1]. The terminal numbering is now consistent with the connections of the transformers.
3. Initial points required for linear models (FOT, FBS) was updated to match the change in terminal numbering of the virtual nodes. Also, the initial points for center-tapped transformer secondary nodes was corrected to introduce the 180 degree phase shift.
4. Added loads/generator models which are connected between two secondary terminals (240V devices) of center-tapped transformers.

[codecov]

Codecov Report

Merging #407 (1681b12) into main (c9d7b87) will increase coverage by 1.23%.
The diff coverage is 98.56%.

@@            Coverage Diff             @@
##             main     #407      +/-   ##
==========================================
+ Coverage   70.14%   71.37%   +1.23%     
==========================================
  Files          59       59              
  Lines       14831    15084     +253     
==========================================
+ Hits        10403    10766     +363     
+ Misses       4428     4318     -110     

Impacted Files	Coverage Δ
src/data_model/utils.jl	67.36% <ø> (ø)
src/data_model/transformations.jl	45.96% <86.66%> (+0.46%)	⬆️
src/form/bf_fbs.jl	96.86% <98.07%> (+0.07%)	⬆️
src/form/fotr.jl	97.27% <98.07%> (+0.03%)	⬆️
src/form/fotp.jl	94.49% <98.18%> (+0.47%)	⬆️
src/core/data.jl	68.49% <100.00%> (+0.64%)	⬆️
src/data_model/eng2math.jl	91.33% <100.00%> (+0.37%)	⬆️
src/form/acp.jl	64.66% <100.00%> (+3.06%)	⬆️
src/form/acr.jl	64.24% <100.00%> (+0.44%)	⬆️
src/form/bf_mx.jl	49.81% <100.00%> (+2.81%)	⬆️
... and 14 more

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[pseudocubic]

I noted some places where some comments should be added in code to signify what that code is used for (to make it easier to find these places in the future).

I also recommend adding a page to the manual as a technical note about center-tap transformers and how we are modeling loads / generators connected to them

[pseudocubic]

Some additional comments about documentation. Something I am trying to going forward is to have mathematical representations of the constraints in docstrings, because going back to fix it is very difficult / time consuming, so that's where a lot of the comments are coming from.

One additional question: in the dss2eng conversion, PVSystems, Loads, and Transformers all have special handling, but what about Generators, and Storage? It seems to me that those objects would exist on the customer side as well (diesel generators, battery backups), so should be supported?