The black hole at the centre of the galaxy algorithm

[thesamovar]

Idea: create a new fixed cluster, cluster 1 that is initialised at the beginning with no spikes, and cannot be deleted (like the noise cluster 0). We force this cluster to have a diagonal covariance matrix which we compute explicitly. We don't compute the correction term. We do have a prior point but no noise point. No change to penalty computation.

This should remove some points that are causing large cluster masks and slowing down the computations.