Fix bugs in low power charging predictions, add automatic battery charge curve feature

What's Changed

• Reduce low power margin to 10 minutes by @springfall2008 in #653
• Low power charging, more accurate setting and some debug by @springfall2008 in #654
• Update customisation.md - car charging hold docs by @springfall2008 in #655
• Fix battery charge curve modelling for lower than max battery charge … by @springfall2008 in #657
• Find charge curve code by @springfall2008 in #658
• Tidy up battery power curve calculations/reporting by @springfall2008 in #659
• Update apps-yaml.md by @springfall2008 in #660

Full Changelog: v7.15.2...v7.15.3