include datestamp functions

[jonathancallahan]

We create a lot of datestamps so might as well regularize it with:

datestampUTC <- function(
  datetime = NULL,
  timezone = NULL, # read from datetime if is.POSIXt(datetime)
  unit = "sec",
  style = "numeric" # or "julian" or "clock"
) 

• "20190909121500"
• "2019252121500"
• "2019-09-09T12:15:00"

Components will be stripped off to support unit. Julian dates will be floored to month if `unit = "month".