Add a reducer model that reduces all the split phase secondaries to their primary

[abodh]

Preliminary Checks

• I have searched existing issues for a duplicate feature request and I'm sure this is new.
• I have read and followed the docs and still think this is missing.

Description

Currently there are two types of reducer:

1. three phase reducer: That reduces all downstream loads to its nearest upstream three phase node
2. primary reducer: that reduces all downstream nodes to its nearest upstream primary node (i.e. kV >= 1 kV)

However, we might also need a reducer that only removes the secondary split phase split_phase_reducer. Reducer 2 removes all the three phase load transformers too as such transformers typically have secondary voltage ratings of 208V, 480V, or 600V which gets filtered out by reducer 2. Reducer 1 can be used but it does not maintain the unbalanced nature of the distribution grid for laterals with phase configurations such as phase A, phase AB, etc.