Add metadata to SimpleNWS

[kamiyo]

Added code to populate metadata in update_forecast() and update_forecast_hourly(), and added properties to get the metadata. The metadata is only populated if response is not empty, in case the API returns metadata with empty periods.

Added tests for the metadata.

Metadata keys are stored in a list, and populated by iterating through the keys; I felt this was the easiest and lowest code way to do it.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 93.79%. Comparing base (5494e54) to head (89503a7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #214 +/- ## ========================================== + Coverage 93.67% 93.79% +0.12% ========================================== Files 9 9 Lines 632 645 +13 ========================================== + Hits 592 605 +13 Misses 40 40 ```

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[MatthewFlamm]

Id to like to see if we need to do this for observations before publishing a release to avoid multiple breaking change releases.

[MatthewFlamm]

Both detailed forecast and observations already return all values under properties. So this PR also harmonized this package. We are good to release a new version if you have no other planned changes.