Name for `tc` objects and general behavior if a `tc` object is not a list

[henningte]

In my point of view, the name "time column" is unintuitive for tc objects because the respective classes the tc class can be built on top do not have to be lists and st_tc() does not enforce that the result is a list. So tc objects really are only lists if the underlying class is a list.

I don't know yet if this will cause problems. Any ideas?

[henningte]

Closing this issue because the tc class is deprecated now.