resolve #2310

* emon: configure ratios without reboot

* settings: serialize() support

* debug: use vsnprintf from newlib, not from sdk

* settings/experimental: show defaults via `get`

* emon: override base methods, fix defaults

* sensor/emon: expose internal index calculation

- refactor configuration to use the correct index when accessing indexed
  sensor methods. store index value on magnitude, refactor loops to
  accomodate this new functionality
- rename slot(index) -> description(index), since we use 'slot' as
  numeric value

- 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.

* Sensors: refactor configuration

- move sensor implementaion to the .ino,
  remove dependency undef / define from sensor files
- update test/build/sensor.h from SENSOR_SUPPORT
- allow to change sensor config variables externally
- `#include <...>` for global headers and libraries,
  fix relative path for math library

* add missing sns <-> i2c dependency

* ledrelay should return relay_none as default

* rollback to original test header

* include debug header when requested (relative)

* Add Shelly 2.5

* Add ADE7953 sensor support

* Fix compilation

* Use only one instance

* WIP: Not working, I got StoreProhibited exception

* Fix exception

* Fix merge issues

* Apply request review changes

* declare struct before using it

* uninterruptible delay

* try optimized reg_size

* fix types

* Remove gpio 0 input in analog sensor

* Add energy to ADE7953, it is not finished yet

* Add new method to save energy with index

* Finish energy for ADE7953

Thanks to @tonilopezmr 
1c3d68af34

* Testing branch

* Added ability (by define) to scale analog sensor, also with number of decimals (it was formerly zero)
Useful when running on batteries to check the battery voltage just 
adding the appropriate resistor

* Reverting DHTSensor.h file

* Trying again to revert DHTSensor.h

* Using macro DEBUG_MSG

* fix method types

* Testing branch

* Added ability (by define) to scale analog sensor, also with number of decimals (it was formerly zero)
Useful when running on batteries to check the battery voltage just 
adding the appropriate resistor

* Reverting DHTSensor.h file

* Trying again to revert DHTSensor.h

* Using macro DEBUG_MSG

* fix method types