- general conversion from .ino modules into a separate .cpp files
- clean-up internal headers, place libraries into .h. guard .cpp with _SUPPORT flags 
- fix some instances of shared variables instead of public methods
- tweak build system to still build a single source file via os environment variable ESPURNA_BUILD_SINGLE_SOURCE

Co-authored-by: Joop Hilverink <33230021+HilverinkJ@users.noreply.github.com>

- Update sensor classes to support a generic way to store energy values
- Update sensor conversion code to deal with units and not magnitudes
- Add magnitude<->unit for sensors, generic way of defining used unit. Convert from sensor magnitude unit to the one used for display.
- Reset energy value based on index through external means (MQTT, HTTP)
- Rework energy timestamping, update webui with 'last saved' value

While this solves the energy conversion issues and we are finally seeing the real value, what I don't really like:
- KilowattHour and WattHour are separate enum tags, thus sort-of are different types altogether
- Conversion code in Energy object should probably use some generic 'ratio' calculation? (https://en.cppreference.com/w/cpp/numeric/ratio/ratio)
- We are still using runtime checks to do calculations and depend that sensor outputs only one specific value type.

Consider this a fix for energy display / storage and preliminary work on sensor.ino
Further sensor refactoring... soon.

new devices&sensors + some fixes