grove/accel3xdigital support for interrupts

[mattetti]

The sensor can trigger interrupts based on analyzed events. The type of events and the pin used to trigger these events can be defined as explained in the datasheet: http://www.farnell.com/datasheets/1670762.pdf

Unfortunately, the current state of exp/io/gpio doesn't allow us to add this feature. That said, we expect the gpio pkg to be soon ready. If you are interested in this issue, look at the gpio pkg, if he does support interrupt pins, then you can work on it.