Handle derived data in cluster drop

[alxvth]

ScatterplotPlugin::loadColors can handle the case when the shown point data and the incoming clusters are derived data.
This occurs for example when you want to drop a cluster data on an HSNE refinement (and the clusters were defined wrt to the point data that the HSNE is based on)

We should allow this case when dropping clusters into the widget using the same number-of-point-checking logic.

Scatterplot/src/ScatterplotPlugin.cpp

Lines 812 to 825 in f3e7e43

	void ScatterplotPlugin::loadColors(const Dataset<Clusters>& clusters)
	{
	// Only proceed with valid clusters and position dataset
	if (!clusters.isValid() || !_positionDataset.isValid())
	return;
	// Get global indices from the position dataset
	int totalNumPoints = 0;
	if (_positionDataset->isDerivedData())
	totalNumPoints = _positionSourceDataset->getFullDataset<Points>()->getNumPoints();
	else
	totalNumPoints = _positionDataset->getFullDataset<Points>()->getNumPoints();
	// Mapping from local to global indices

[alxvth]

I've cherry-picked this into the release/core_bican_bg/bican_bg branch so that it can land in an installer. (One of the bican projects needs this fix)