Add unit tests to HPWHsim; change names of library and a few functions

[spahrenk]

Description

• HPWHsim unit tests were converted to using googletest. In the process, the library name was changed to HPWHsim and the names of a few functions used by cse were changed.
• Changed dhwloop32U.rep ref due to 0<->-0 differences.

Author Progress Checklist:

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• Iterate with author until all changes are approved

[nealkruis]

Thanks, @spahrenk!

This is looking good and can drop in after we settle out some of the other open PRs.

[nealkruis]

@chipbarnaby this PR is likely ready to drop in (after the corresponding HPWHsim branch is merged). It includes some minor API changes for HPWHsim (i.e., removes redundant naming redundancies).

[nealkruis]

The corresponding HPWHsim PR has been merged into its main. @spahrenk can you:

1. Merge CSE 'main` into this branch
2. Update the HPWHsim submodule on this branch to point to its main?

Thanks!