add clause for monitoring hierarchy (#124)

[tomkralidis]

Fixes #124

[solson-nws]

Looks good

[josusky]

As discussed during today's TT-WISMD teleconference, here are may first thoughts (slightly more composed actually):

• Using topics like monitor/a/wis2/ca-eccc-msc-global-discovery-catalogue/fr-meteo-france means that subscribers who now put wildcard (*) as the first element of the topic (because they do not care if the data is in the cache or not) will need to adjust. It is not a big issue, just a certain discomfort, not only now but also in the future.
• If I understand it right that the payload is not going to be WNM then it is questionable if it shall be in WTH because the two have been (until now) considered coupled. That is includes also the version part of the WTH.
• The fact that the level 6 is not going to be data-policy is also not very nice.

To be constructive, what about keeping the first levels without change (probably as origin or would there be a caching of it?) and just adding a new value for the level 5 (notification-type)? At this level all the (data) subscribers use data and the GDC listens to metadata so a third option monitoring (or report, or monitoring-report?) should not cause any confusion. I am still not fully OK with misusing the level 6, but I agree that putting there some bogus value is not nice either.
BTW, why do we need both centre IDs, originator of the report as well as the target/subject of the monitoring, in the topic? As a center being monitored, I would probably want reports from all Global monitoring centres, so perhaps origin/a/wis2/my-centre-that-is-monitored/monitoring-report would be enough (Not perfect, I know, the meaning of centre-id is kind of reversed here).