Remove centers magic numbers

[merrygoat]

11A and 13A centers can be output to xyz files. The variables to do this are set using magic numbers which are unstable to changing the cluster lists.

Only the centers of a limited number of clusters are interesting - not all clusters even have centers.

There are multiple ways of fixing this:

• Output centers of all clusters using the "S" of the 's' clust lists. This would produce a centers xyz file for each of the cluster types. While this would be messy (many empty files) it would also be the most general (all clusters with centers and all future clusters would be output).
• Iterate over the cluster name list to identify the position of 11A and 13A to set the magic numbers at runtime. This reduces the files output and is most like the original behavior. The disadvantage is that outputting the centers of other cluster types would not be supported without further changes.
• Remove all center xyz output and make sure that the centers are output as a separate species in the full cluster xyz files. A python script could be supplied to strip the centers from the xyz and make center xyzs if required. This is the cleanest method and reduces the number of output types, however for people processing cluster centers this increases the number of steps required and complexity to get the data.

Which is best depends on how much the xyz centers are used and whether the centers of any other clusters are interesting.

[tranqui]

Would not a cluster-independent method of doing this be to output the center of masses for each cluster? Edit: Care would need to be taken with respect to periodic boundaries, by e.g. unwrapping coordinates when a cluster crosses a boundary.

[merrygoat]

After discussion, the second option seems the most sensible for now.

Larger scale restructuring of the output format is something for a future version > 1.0.