Output coverage database path has changed

[nickg]

Previously for a top-level unit called top in a library lib the coverage database would be written to lib/_LIB.TOP.elab.covdb, now it is written by default to top.ncdb in the current working directory, and the path can be configured with the new --cover-file= option when elaborating.

There is a redirect from the old name so any existing scripts should continue to work, although this may be removed in a future release.