Define "geographic coordinates"

[mgiuca]

The spec uses the term "geographic coordinates" without defining it or referencing another spec:

The latitude and longitude attributes are geographic coordinates specified in decimal degrees.

I noticed that altitude is defined in terms of the WGS84 spec but lat/long are not. The "geographic coordinates" presumably refer to standard latitude/longitude coordinates in degrees, but it would be good to explicitly specify these, so it's clear:

• What + and - lat means (presumably N and S respectively).
• What + and - long means (presumably W and E).
• What 0 long means (presumably London).

I assume this is defined in WGS84 so it would be good to simply reference that.

[reillyeon]

The non-normative Scope section says that the position information is provided in terms of WGS84, but only the altitude property is normatively defined in terms of those format. The latitude and longitude properties should also be normatively defined as WGS84 coordinates.

[marcoscaceres]

Closed by #138