Methods for `sftime` objects

[henningte]

Methods for sftime objects to add:

• [x] rbind.sftime
• [x] cbind.sftime
• [x] [.sftime
• [x] [[<-.sftime
• [x] $<-.sftime
• [x] st_time.sftime #4
• [x] st_time<-.sftime
• [x] st_set_time.sftime
• [x] st_drop_time
• [x] st_transform.sftime
• [x] st_as_sftime.ST
• [x] st_as_sftime.data.frame
• [x] st_as_sftime.sf
• [x] st_as_sftime.stars
• [x] st_as_sftime.sftime
• [x] st_as_sftime.Track
• [x] st_as_sftime.Tracks
• [x] st_as_sftime.TracksCollection
• [x] st_as_sftime.sftraj #1
• [x] st_as_sftime.sftrack
• [x] methods for geometric operations from the sf package which do not directly work for sftime objects
• [x] st_join.sftime
• [x] st_filter.sftime
• [x] tidyverse join methods
• [x] Other tidyverse methods.
• [x] Conversion from spatstat.