feat: add function to create transformers within substations

[danielolsen]

Pull Request doc

Purpose

Closes #206. This is a draft PR right now, since it relies on code from #210. It could also become part of #210.

What the code is doing

Using buses, grouped by substation, for each substation with more than one bus, a transformer is added between the highest-voltage bus in the substation and each other bus. This is an arbitrary topology that seems reasonable enough, although it is by no means the best one.

Testing

A new unit test is added which covers the bases of: no transformers within a substation, one transformer within a substation, multiple transformers within a substation.

Usage Example

>>> from prereise.gather.griddata.hifld.data_process.transmission import (
...     build_transmission,
...     create_buses,
...     create_transformers,
... )
>>> lines, substations = build_transmission()
dropping 6892 substations of 70857 total due to LINES parameter equal to 0
dropping 759 lines with one or more substations listed as 'NOT AVAILABLE' out of a starting total of 71554
dropping 3143 lines with one or more substations not found in substations table out of a starting total of 70795
Evaluating endpoint location mismatches... (this may take several minutes)
dropping 120 lines with one or more substations with non-matching coordinates out of a starting total of 67652
dropping 143 lines with matching SUB_1 and SUB_2 out of a starting total of 67532
2119 line voltages can't be found via neighbor consensus
310 line voltages can't be found via neighbor minimum
>>> bus, bus2sub = create_buses(lines)
>>> transformers = create_transformers(bus, bus2sub)
>>> transformers
      from_bus_id  to_bus_id
0               0          1
1               2          3
2               5          6
3              12         13
4              20         21
...           ...        ...
5394        58235      58236
5395        58239      58240
5396        58246      58247
5397        58258      58259
5398        58262      58263

[5399 rows x 2 columns]

Time estimate

15 minutes.

[danielolsen]

With #210 merged, this is now ready for a full review.

[rouille]

LGTM