True if and only if the units are longitude units.
This is the case if the units attribute is one of 'degrees_east', 'degree_east', 'degree_E', 'degrees_E', 'degreeE', and 'degreesE'.
Returns False if the units attribute is 'degrees'.
Examples
>>> u = Units(units='degrees_east').islongitude
True
>>> u = Units(units='degrees').islongitude
False
>>> u = Units(units='degrees_north').islongitude
False