sensors without FIFO

[PaulStoffregen]

availableGyroscope() should be required.

Sensors without a FIFO should have their available functions return 1 when the sensor has produced a new measurement since the most recent read, or 0 if no new measurement has been completed where reading again would give duplicate data.

Filter algorithms which accumulate need to be able to know whether the read is the same data they previously used, so they don't double count anything. This is always done with gyroscope data, so it's really important for the API to not allow a default 1 return for availableGyroscope(). Some advanced algorithms may also accumulate accelerometer data, but Madgwick does not. As far as I know, this isn't important for magnetometer data.

The sampleRateGyroscope() function should probably also be required, without a default. The data just isn't usable if this info is missing.

[PaulStoffregen]

For accelerometers and magnetometers, we should probably allow the defaults.