Check if we are operating in a grouping before issuing a warning

Check if we are operating in a grouping before issuing a warning

If we are examining a list in a grouping, we should not be issuing
a warning, as we do not know where that grouping is going to be
instantiated.

Once we determine that a warning may be appropriate, walk statement
contexts upwards, searching for a grouping -- if we find one, just
bail out without any warning.

JIRA: YANGTOOLS-957
Change-Id: If54b05cc444f19f56c6f8b4f931d85b0b270b4ac
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>