arctan2 fails to account for units #890

When calling cf.Data.arctan2(y, x), the function will ignore any units that y and x have and just treat them as unitless.

This means, for instance, that if y is in metres and x is in kilometres, no conversion will take place to divide the values of y by 1000 before calculating da.arctan(y, x), resulting in incorrect values being returned due to incorrect unit scale in the same dimension.

If x and y have units, cf.Data.arctan2() should attempt to conform the units if they have the same dimension and raise an error if they do not.

If either x, y, or neither have units, then cf.Data.arctan2() should treat them both as unitless.

import cf

y = cf.Data([1], units=cf.Units('km'))
x = cf.Data([1], units=cf.Units('m'))

rad = cf.Data.arctan2(y, x)
print(rad.array)

[0.7853981633974483]

vs.

import cf

y = cf.Data([1000], units=cf.Units('m'))
x = cf.Data([1], units=cf.Units('m'))

rad = cf.Data.arctan2(y, x)
print(rad.array)

[1.5697963271282298]