Changed maxlevel to 4 in checksums files
As @clemenci found out, dumping the checksums for all the nodes in the geometry is not viable once all detector are in place: we end up with millions of lines in the checksum file...
We therefore propose to limit the checksums to 4 levels down the hierarchy to at least be able to identify the subtree that has changed (this has no influence of the checksums themselves, they are done taking the full tree into account, we just dump the top nodes). A command to generate the file with the correct option has been added, and the documentation updated.