Plan optimisations to reduce random exports & charges

What's Changed

In an attempt to reduce random export slots which have little impact the default metric_min_improvement_export has been changed to 5, meaning export must be worth at least 5p/30 minute period to be triggered. At a 3kw export rate this would be 5p for 1.5kWh which is around 3.3p per kWh. You can change this setting in expert mode.

Changed the minimum export slot size to 5 minutes to allow for shorter slots thus preventing false selection of slots where the load is higher in an attempt to get a shorter slot by the backdoor.

Tweaked the metric keep calculations to stop force charges to keep the battery level above keep when its not profitable to do so.

• Unit tests by @springfall2008 in #1711
• Changes to try to prevent random bad exports by @springfall2008 in #1714
• Potential planning improvements by @springfall2008 in #1719
• Tidy up export slots by @springfall2008 in #1720
• Default metric min improvement export increased by @springfall2008 in #1724

Full Changelog: v8.8.4...v8.8.5