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

This addresses some 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~~ some
of the annoyingly flaky restart tests. ~~I'll fix this before I assign it.~~

This probably shouldn't be merged into the branch before #996.