Fixes for metric keep & iboost scaling

What's Changed

Iboost value scaling

New input value in iboost model which controls how the value of iboost is accounted for - default to 0.75x meaning each kWh of iboost would be valued at 0.75 x lowest_import_rate.

Bug fixes

Fix issue with metric keep causing export during car charging.

• Fixes for metric keep & iboost scaling by @springfall2008 in #1277
  Full Changelog: v8.2.2...v8.2.3