split: Target del_split split in two parts

Phony target .del_splitdir allows to only remove
the directory splitdir in case it is empty.
cat: Symlinks to single trajectories in splitdir

If splitdir exists, create symlinks from splitdir to catdir.
Before, ALL files in splitdir with a basename that also occurs
in catdir are removed, e.g. basename-01 and basename-100.
split: Properly treat comment lines

Comment lines are still printed out, but
they now do not affect the partitioning.
split: Do not print follower column in split data
cat: Phony target to create links in splitdir