Further plan optimisations, solar assistant template

What's Changed

Minor fixes related to export planning

NOTE If you use expert mode its recommended you change metric_min_improvement_export to 5

• Fixes related to export planning by @springfall2008 in #1725
• Unit test fixes by @springfall2008 in #1726

Solar assistant template

• Add solar assistant to predbat by @xhemp in #1723

New Contributors

• @xhemp made their first contribution in #1723

Full Changelog: v8.8.5...v8.8.6