Added getYPR, getYPRRadians and getQuaternion. Possible usage for variable integration steps.

[afrancescon]

Added convenient method to access yaw, pitch and roll in one call (helps in some to keep code clear).

Added getQuaternion to access internal quaternion where needed.

[afrancescon]

I further added update methods with time (in millis) argument. This can be helpful when integration time is not fixed but can vary due to other MCU activities. I put attention to keep signature back-compatible.

[facchinm]

There are a couple of typos (like ellapsedTime -> should be elapsedTime).
The signatures on keywords.txt do not match the actual functions (getYPR -> getRPY).
About the latter, we usually avoid providing APIs that accept pointers, so it would be better to pass the values by reference (or creating a RollPitchYaw object and returning it if "atomic" operations is required)

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