[mlir][sparse] Correcting some terminology / naming-scheme errors.

The name "coords" should be used for the complete tuple of Dimension-/Level-many "crd" values associated with a single element.  Whereas the name "coordinates" should only be used for collections of "crd" values which span several elements (e.g., the tensor's coordinates buffer for a single level).

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D147291