#996 feedback - rename "tag", "c_time", "ctime" to "point", "point_string"

This addresses feedback on #996 for #119.

This renames all references to `tag` or `c_time` or `ctime` to `point` or `point_string`,
as appropriate. `point_string` is used where a string representation is used, and
`point` is used when a `cylc.cycling.PointBase`-derived object is used.

At the moment, this fails the `insert task with stop point` functionality and the
annoyingly flaky restart tests. I'll fix this before I assign it.

This probably shouldn't be merged into the branch before #996.