See https://github.com/pinchies/Marlin/pull/1#issuecomment-471388743

Mostly from the L6470-oriented PR

* Apply #pragma once in headers
* Adjust some thermistors formatting
* Misc cleanup and formatting

- Clean up the API to use a `static` class instance to adhere to Marlin convention
- Add `const` position data access for read/write
- Add Storage capacity to the interface

The current Marlin implementation relies on a timer interrupt to start the ADC conversion and read it. However in some circumstances the interrupt can be delayed resulting in insufficient time being available for the ADC conversion. This results in a bad reading and false temperature fluctuations. These changes make sure that the conversion is complete (by checking the ADC hardware via the HAL) before reading a value.

See: https://github.com/MarlinFirmware/Marlin/issues/11323