Disk Read/Write Move and Rename

Renamed 'write-to-disk' to 'write-fringe-to-disk' and read-from-disk to read-fringe-from-disk
correspondingly.  Also moved these two functions to stp-solve-base.rkt, removing them from
stp-solve-cluster.rkt and stp-solve-single.  Finally, changed write-fringe-to-disk so that it
writes one position per line to make it more human readable during debugging.