The CF coordinate type.
One of 'T', 'X', 'Y' or 'Z' if the coordinate object is for the respective CF axis type, otherwise None.
Examples
>>> c.standard_name
'longitude'
>>> c.ctype
'X'
>>> c.units
'days since 2005-12-1'
>>> c.ctype
'T'
>>> c.units
'kelvin'
>>> print c.ctype
None