Time continuous KML

Write a time-continuous line data set to KML, the function write_track_kml creates a KMZ file which can be opened in Google Earth. Use the time slider to control the range.

Documentation for the function: http://trackage.github.io/trip/reference/write_track_kml.html

• support various colouring schemes
• support elevation and the various altitude modes
• integrate into trip (or appropriate format-writer)

Need at least vectors of id, lon, lat, utc (date-time in POSIXct), z (optional altitude), and can also supply kml_file and name, but an auto-temp file is created if these aren't supplied.

Return value is the KMZ (or KML) file name.

## the latest verion trip is required (1.6.0 at least)
##remotes::install_cran("trip")
library(trip)
write_track_kml(id, lon, lat, utc)