Screen and Device local coordinates

[pozdnyakov]

This patch introduces definition for 'device' and 'screen' coordinate systems as possible local coordinates options for spatial sensor classes.

The SpatialSensorOptions dictionary and the "construct spatial sensor object" are added, so that the client can set the local coordinate system for a Sensor instance from the constructor.

This patch is a part of fix for https://github.com/w3c/sensors/issues/257

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[pozdnyakov]

@kenchris, @alexshalamov, @anssiko PTAL

[pozdnyakov]

@kenchris I've updated the patch, could you take another look?

[kenchris]

"In physics, a frame of reference (or reference frame) consists of an abstract coordinate system and the set of physical reference points that uniquely fix (locate and orient) the coordinate system and standardize measurements." (wikipedia)

I don't think it necessarily includes times, though I does in Einsteinian relativity.

In a way, a 4 component system with time included is also a coordinate system

[kenchris]

Found this online: https://www.physicsforums.com/threads/frame-of-reference-vs-coordinate-system.110090/

[pozdnyakov]

PTAL at the updated patch

[pozdnyakov]

From https://www.quora.com/What-is-a-reference-frame-and-how-does-one-determine-where-a-frame-of-reference-begins-and-where-it-ends :

A reference frame is really just a coordinate system for spacetime, i.e., history. It has to take every event in history and assign it time (t) and space (x, y, z or the like) coordinates.

[kenchris]

Lgtm

[pozdnyakov]

Thank you for review!