CFtime 1.5.0

• Code is updated to align with the CF 1.12 release. Specifically, calendars
  utc and tai are added.
• standard calendar now uses mixed Gregorian/Julian calendar as defined in the
  CF Metadata Conventions. proleptic_gregorian is now a separate calendar with
  its own code base.
• Negative offsets from a calendar origin are allowed.
• Code is refactored to R6. R6 class CFTime replaces S4 class CFtime (note the
  difference in case). S4 class CFdatum has been replaced by hierarchy of
  R6 CFCalendar classes, with various non-exported functions converted into
  methods of CFCalendar. The code is now much cleaner and easier to extend, such
  as with the two new calendars utc and tai.
• ISO8601 timestamps can use a comma "," as decimal mark to separate a
  fractional part from a time element, in addition to the dot ".".
• Do not drop degenerate dimension on bounds when only 1 offset is included in
  subsetting.