Add Naming Consistency, Customization Tests

[amzhao16]

This PR is a couple things combined, but mostly correctness issues that I noticed while writing:

1. plurality consistency - because I do add_load_nodes it makes sense to keep plurality with add_generators
2. update default values for industrial load (based on ieee13 standards, i think i just had an updated placeholder before)
3. remove customization of Conns: doesn't match the type of (str, int) that all other customizations do, and also wouldn't make sense as is (user would have to internally poke the Connections class?)
4. add missing customization tests for generators/pv systems/transformers

[sarahmish]

since sim.json is part of the repository, I would not include it in the .gitignore. there is a potential that sim.json would be changed in the future.

[amzhao16]

sim.json changes each time I run the test cases, because of the randomness assigned to them - I'll rewrite the test case so it doesn't change each time