refactor: Improve reliability and simplify implementation
call HAL_RTC_Init() only when needed to avoid losing hundreds
of milliseconds on RTC counter
remove useless check of _configured at the beginning of
STM32RTC class methods
Use flag "INITS" to detect whether RTC calendar is initialized
(except for F1 which doesn't have this flag)
This prevent the use of year 2000 which is the default hardware
reset value and doesn't trigger INITS flag
Use Backup registers only for F1, and no more for other series
Simplify class state management:
remove _configured and _reset, _alarmEnabled
add _timeSet
Tested on BluePill and Nucleo L476RG, with examples from STM32duino_RTC library
refactor: Improve reliability and simplify implementation
Tested on BluePill and Nucleo L476RG, with examples from STM32duino_RTC library
fixes #66