Bugfix: Load following heuristic method for variable load signals only using the beginning of the load signal

[genevievestarke]

Fixing a bug in the heuristic load following dispatch method that affects variable load following signals.
Prior implementation incorrectly assumed flat load profile even in the event of a variable profile; this PR fixes that so it correctly handles variable signals.

PR Checklist

• RELEASE.md has been updated to describe the changes made in this PR
• Documentation
  • Docstrings are up-to-date
  • Related docs/ files are up-to-date, or added when necessary
  • Documentation has been rebuilt successfully
  • Examples have been updated
• Tests pass (If not, and this is expected, please elaborate in the tests section)
• PR description thoroughly describes the new feature, bug fix, etc.

Related issues

Impacted areas of the software

• hopp/simulation/technologies/dispatch/hybrid_dispatch_builder_solver.py

Additional supporting information

NA

Test results, if applicable

[jaredthomas68]

I'd like to get this in ASAP for P2N. Only thing I see missing is that there are no tests around this.

[johnjasa]

I see this is a draft but you also requested review, @genevievestarke; are you planning to add more to this?

[genevievestarke]

@johnjasa I was hoping to put a test in, but also using this as a chance to go through the new pull request structure.