add timezone to pv yield

[branberry]

Pull Request

Description

This pull request is for issue #127: Make sure pv yield has timezone added to it. Adds timezone to PV yields when calling the get_pv_yield function.

Fixes #

How Has This Been Tested?

Ran unit tests as described in the readme. All tests passed succesfully.

• [x] Yes

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked my code and corrected any misspellings

[peterdudfield]

Thanks @branberry for doing this issue