Add Point object to SDF format

**Original report ([archived issue](https://osrf-migration.github.io/sdformat-gh-pages/#!/osrf/sdformat/issues/201)) by Anonymous.**

The original report had attachments: [SDF_PhysicalObjectsSpecification.pdf](https://osrf-migration.github.io/sdformat-gh-pages/data/repositories/osrf/sdformat/issues/201/attachments/SDF_PhysicalObjectsSpecification.pdf)

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Previously E-mailed May 15, 2017 to Nate Koening (OSRF) from Paul Mitiguy.

From 2017 meetings about SDF format/parsers with input from Nate Koening (OSRF), Adam Leeper (Google), Terry Denery (Mathworks), Paul Mitiguy (TRI/Stanford).

If this has not already been done,
please add a Point object (described below) as a physical object in SDF format.

A point is a 0-dimensional object having translational properties, e.g.,
position from another point, velocity, acceleration, jerk, and force.
A point does not have rotational properties, e.g., a point does not have orientation or angular velocity.

A point may or may not be part of (physically welded to) a rigid body
If part of a rigid body, it must be apparent in the .sdf format.

Points may be the origin of a frame, the center of mass of a body, a point on a rigid body where a force is applied, or a convenience for tracking out the path (trajectory) of contact of an object with another object.

Note: A point may represent a computed location such as the center of mass of a system of bodies and/or particles and may be transient such as a contact point between two bodies.