CFtime 1.3.0

Changes since release 1.2.0:

• Two CFtime instances can be added if they have compatible calendars and units.
  The earlier origin is preserved in the result and offsets from the later instance
  are incremented by the appropriate amount. As before, in the result offsets are
  in the order of the specified CFtime instances.
• A new function CFsubset() can be used to retrieve a logical vector that indicates
  which time steps in a CFtime instance fall between two timestamps. This is useful
  to slice data during reading from file or analysis.
• Time zone information is managed at the level of the datum. If a vector of character
  timestamps is parsed and found to have different time zones, a warning is generated.
• Much reduced memory footprint.
• Minor code fixes, see GitHub commits.
• Codecov.io test coverage monitoring added.
• Documentation updated, with description of new functions.