Bug in pysiaf.utils.rotations.unit_vector_from_cartesian

[sanjibs]

Running

import pysiaf.utils.rotations
pysiaf.utils.rotations.unit_vector_from_cartesian(x=0.0, y=0.0)

incorrectly throws the following error
"UnboundLocalError: local variable 'unit_vector' referenced before assignment"

The problematic line is 778 in pysiaf/utils/rotations.py which seems to be checking for valid values but in turn ignores the valid value of zero.

if np.array([x_rad]).all() and np.array([y_rad]).all():
    unit_vector = np.array([x_rad, y_rad, np.sqrt(1-(x_rad**2+y_rad**2))])

Instead should be replaced with something like

if z is None:
    unit_vector = np.array([x_rad, y_rad, np.sqrt(1-(x_rad**2+y_rad**2))])