Code speedup, comments and support for multiple files during update

What's Changed

Mostly internal optimisations and cleanup.

• Supporting nordpool format rates by @springfall2008 in #697
• Support multiple files for Predbat internal update feature by @springfall2008 in #701
• Doc strings by @springfall2008 in #704
• Code optimisations by @springfall2008 in #705

Full Changelog: v7.15.11...v7.15.12