Make st_get_time() similar to st_geometry()

[henningte]

st_get_time() currently only extracts the time column. It may be useful to transform this function into a method to get or set the active time column in an sftime object similar to how st_geometry() can be used to get or set the active geometry column in an sf object.

I'm not sure how to name such a generic to avoid conflicts with the other packages. Perhaps st_tc_column()?

Similar to sf, a st_get_time() function could remain in sftime.

[edzer]

st_time()? (with an st_time<-() method so that st_time(obj)<-"time_col" sets it, and an st_set_time() alias for use in pipes)

[henningte]

This should work now.