clarify that coordinates indicate gridpoint location

[jonathan]

In view of the problem raised by Thomas Lavergne on the email list in the thread "X and Y projection coordinate (center vs corner)", I propose that we insert the following clarification at the end of the introduction to section 4 of the CF standard:

The values of a coordinate variable or auxiliary coordinate variable indicate the locations of the gridpoints. The locations of the boundaries between cells are indicated by bounds variables (see section 7.1 on "Cell Boundaries"). If bounds are not provided, an application might reasonably assume the gridpoints to be at the centers of the cells, but we do not require that in this standard.

I don't believe that inserting this text would change the intention of the CF standard.

[mlaker1]

Defect has been fixed.