pvl_singleaxis error at low sun angles

[cwhanse]

Describe the bug:

Low sun angles with a tilted tracking axis cause an error in the backtracking calculation. Code attempts arcos() of a value outside [-1, 1]
pvlib-python #656

To Reproduce

SunZen=80;
SunAz=338;
Latitude=nan;
AxisTilt=30;
AxisAzimuth=180;
MaxAngle=60;
GCR=0.35;

Backtrack=0;
[~,~,mod_thetam, mod_azm]= pvl_singleaxis_v2(SunZen, SunAz, Latitude, AxisTilt, AxisAzimuth, MaxAngle, Backtrack, GCR)

Backtrack=1;
[~,~,mod_thetam, mod_azm]= pvl_singleaxis_v2(SunZen, SunAz, Latitude, AxisTilt, AxisAzimuth, MaxAngle, Backtrack, GCR)