Improve transmission coverage of HIFLD dataset when source data are missing

# :rocket:

- [x] Is your feature request essential for your project?

### Describe the workflow you want to enable
I wish we had a way to add transmission lines to areas where there it is known to be missing.

### Describe your proposed implementation

One relatively simple way to achieve this could be a user-supplied list of substation coordinates and transmission lines connecting them, which would get appended to the original HIFLD data before further processing. An algorithm to create synthetic information would be nice, but much more difficult.

### Additional context
See New York City: