pvl_singleaxis produces NaN surface tilt angles near solar noon

[DanRiley]

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pvl_singleaxis can produce NaN results for the surface tilt angle when the tilt is 0. Error stems from the following line: SurfTilt = 90 - acosd(dividend./divisor);

When divisor is 0, the division by 0 produces NaN.

Possible fixes could be setting divisor to a very small number (eps) when it is 0, or simply setting SurfTilt to 0 any time that divisor is 0.