Finish computing intersection of time periods

[JackKelly]

Pull Request

Description

This is a bite-sized part of a larger issue (#223). This is the last PR for #223. After this PR is merged, #223 will be done.

The ultimate aim of #223 is to change the way nowcasting_dataset computes the set available datetimes across all DataSources.

This PR implements these features:

• Implement a DataSource.get_contigous_t0_time_periods() -> pd.DataFrame which goes through each period and chops off history_duration from the beginning of the period, and chops off forecast_duration from the end of the period.
• Enable NowcastingDataModule to compute the intersection of all the lists of t0 time periods from each DataSource. Use nd_time.intersection_of_2_dataframes_of_periods().
• Compute t0 datetimes across all those periods (using a user-specified frequency, e.g. '5 minutes').
• As before, split those t0 datetimes into train, valid, test
• Remove nd_time.get_start_datetimes(), nd_time.intersection_of_datetimeindexes(), DataSource.get_t0_datetimes(), and their tests, and use grep to check they're not called from anywhere I've missed.

Fixes #223

How Has This Been Tested?

New tests have been written. All tests pass.

• No
• Yes

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

[jacobbieker]

LGTM!