True if and only if the units are latitude units.
This is the case if the units attribute is one of 'degrees_north', 'degree_north', 'degree_N', 'degrees_N', 'degreeN', and 'degreesN'.
Returns False if the units attribute is 'degrees'.
Examples
>>> Units(units='degrees_north').islatitude
True
>>> Units(units='degrees').islatitude
False
>>> Units(units='degrees_east').islatitude
False