Load internal DomainValues table when service unavailable

[jbousquin]

If ATTAINS domain list unavailable; returns internal using (see line):

obj = load(file = "inst/extdata/DomainValues.rda")

This works, but relies on the current workspace, which may depend on how the library is loaded. system.file() should do the same and will look for the file in the installed package's directory structure, where the contents of the inst/ folder have been moved to the top level.

This is an edge case (handling fallback), noticed on TADA test-run