Make time filtering in PVNet datasets more flexible

[dfulu]

Pull Request

Description

This pull request enhances the PVNet dataset classes by allowing users to filter data using multiple time periods rather than just a single start and end time. It introduces a new utility function get_time_periods_mask() for time period masking and updates both the dataset logic and associated tests to support and validate this more flexible approach.

This makes it easier to perform cross-validation with the dataset. However, note that this is a breaking change and will require corresponding changes in PVNet

Checklist:

• My code follows OCF's coding style guidelines
• 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
• I have checked my code and corrected any misspellings

[Sukh-P]

Looks good, thanks for adding this in!