BUG: Fix geographic coords convert_to_cartesian with units

[chrishavlin]

Close #4892

[chrishavlin]

just confirming, the code snippet from the issue now runs as expected

import numpy as np
dshp = (16,16,16)
data = {'density': np.random.random(dshp)}

bbox = np.array([[0, 1000], 
                [-150., -100.], 
                [-90., 90.]])
ds = yt.load_uniform_grid(data, dshp, geometry='geographic', 
                          bbox = bbox,
                          axis_order=('altitude', 'longitude', 'latitude'))

yt.SlicePlot(ds, 'altitude', 'density', window_size=(3,2))

[chrishavlin]

pre-commit.ci autofix

[chrishavlin]

I'd prefer if we used colatitude instead of theta internally,

good idea. would save me from adding comments to remind myself later that theta is colatitude :)

I'll make those changes.