MNT: rename cluster to fsl-cluster to avoid naming conflicts with third party packages.

We have known for a long time that $FSLDIR/bin/cluster collides with the graphviz cluster command. With the move to using conda for distribution/installation, we will now potentially be sharing the $FSLDIR/bin/ namespace with graphviz. So cluster is being renamed to fsl-cluster.

A wrapper script called $FSLDIR/share/fsl/bin/cluster will be maintained for compatibility with older releases, so that end users and third party scripts can still call cluster.