Cleanup old science values and improve initial subscription

[chapulina]

This addresses this comment by @mabelzhang : #93 (comment)

• Needs this to reduce warnings, but not for functionality: Offer a way to disable warnings on marker manager gazebosim/gz-gui#326

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• Added services for all FloatVs, which were missing. This is needed to get the data on demand, since Ignition Transport doesn't support latching.
• Make the FloatV optional. This is not a use case here, but I think it's a nice-to-have when we upstream.
• Instead of skipping NaNs when displaying, send a DELETE_MARKER command, so that if the data changes from valid to NaN it disappears.
• Removed the - from the marker namespaces because it looked awkward when there's no float topic

[mabelzhang]

Works as intended. I see what you mean by the warnings.
So I'm actually unclear on why they have to be deleted again. I thought ClearMarkers() is called in OnFloatVTopic(), so shouldn't all the points already get deleted whenever the topic is switched?

[mabelzhang]

Will let you merge since we have some specific order of merging at the moment.

[chapulina]

I thought ClearMarkers() is called in OnFloatVTopic(), so shouldn't all the points already get deleted whenever the topic is switched?

Good point, maybe it's a timing thing? I should dig more later. I'll merge this now to unblock other PRs.