Hiding isForSleep parameter in WatchdogSAMD::enable()

[ulysse314]

Since isForSleep parameter in WatchdogSAMD::enable() method should not be used by users of this library (see the comment utility/WatchdogSAMD.h), this patch uses C++ mechanisms to make this parameter invisible by developers, without changing the behaviour of SleepyDog.

This is done by replacing (and by removing the comment related to isForSleep):

public:
  // Enable the watchdog timer to reset the machine after a period of time
  // without any calls to reset().  The passed in period (in milliseconds)
  // is just a suggestion and a lower value might be picked if the hardware
  // does not support the exact desired value.
  // User code should NOT set the 'isForSleep' argument either way --
  // it's used internally by the library, but your sketch should leave this
  // out when calling enable(), just let the default have its way.
  //
  // The actual period (in milliseconds) before a watchdog timer reset is
  // returned.
  int enable(int maxPeriodMS = 0, bool isForSleep = false);

By:

public:
  // Enable the watchdog timer to reset the machine after a period of time
  // without any calls to reset().  The passed in period (in milliseconds)
  // is just a suggestion and a lower value might be picked if the hardware
  // does not support the exact desired value.
  //
  // The actual period (in milliseconds) before a watchdog timer reset is
  // returned.
  int WatchdogSAMD::enable(int maxPeriodMS = 0);
private:
  int WatchdogSAMD::enable(int maxPeriodMS, bool isForSleep);