Implement stm32 ADC continious mode

[forGGe]

According to the ADC app note (see section 1.3
Single-channel continuous conversion mode) and stm32 RM there is a operation mode when new conversion will start immediately after end of previous one.

Support of this mode must be implemented in the stm32 ADC driver and exposed as additional ADC methods, smth like void start(handler &) and stop() that will start or stop continuous conversions.

Following rules apply:

• If external trigger is used, then each new conversion series should start as soon as trigger hit.
• If external trigger is used and special parameter (start align mode) is set, then then 1st conversion series will start when trigger hit and any consequent conversion series should start immediately after previous conversion have been completed.
• If software trigger is used, then 1st conversion series will start during start() call, any consequent conversion series should start immediately after 1st conversion completed.
• Handler must be called at the end of each conversion series. Half-complete events are useful too.

Requires:

• - Implement external trigger support for stm32 ADC #199 External trigger for ADC