Avoid removing and adding graph node when updating properties

[astrofrog]

At the moment, whenever any aspect of a layer is changed, we remove the graph node from OpenSpace and add a new one (this is done in layer_artist.py). We should make sure that we only add the graph node once and then make updates by setting only the properties that need to be set. This can include:

• Changing the data to be shown (e.g. if a subset changes, or the attributes for the RA/Dec change)
• Color
• Visibility
• Transparency
• Point size/brightness