CoreReconstruction: removed N >= 2 requirement

This was necessary in the simulations, but causes trouble in actual data.
Fixed cluster CoreReconstruction

CoreReconstruction now works correctly for a cluster of stations.  It
iterates over the coincidences table and reconstructs the core position
for N >= 3.  The results are stored similarly to the results of the
ClusterDirectionReconstruction table.
Merge branch 'core_reconstruction'