Tracking of p/kWh

What's Changed

Predbat now tracks average p/kWh each day, will show in attributes in cost_today/cost_yesterday/cost_today_car/cost_yesterday_car/cost_today_import/cost_today_export and also on the Web Interface 'Rates' chart.

• Add p/kWh to cost today, remove debug output in futurerate by @springfall2008 in #1580
• Divide by zero fix by @springfall2008 in #1581
• Rename p/kWh at add average by @springfall2008 in #1582
• Put back timing of yesterday data by @springfall2008 in #1583

Full Changelog: v8.5.4...v8.5.5