Remove Load in favor of LoadZIP

[lukelowry]

Summary

Load does not have current injection as an internal state variable and is also a subset of the LoadZIP model.

Rationale

Standardizing on an interface with the network and current injections as state variables internally.

Description

See above

Additional information

None

[lukelowry]

Per @pelesh at the beginning of the technical discussion meeting. The only other component without an internal state for the injection current is Branch, but that is quite a large refactor.

[nkoukpaizan]

It seems simpler to me to update the Load implementation to match the documentation than to nuke it. I'll look at the branch refactoring as well.