Fix weighted sum of squares for the position angle in psfvary.py

[AlexKurek]

Bug
When calculating the weighted sum of squares for the position angle (stdpa), the bpa term was left linear instead of being squared. Because position angles wrap every 180 deg., the code applies a squared correction polynomial for angles $\ge 90^\circ$. Mixing a linear term with squared correction terms results in an invalid variance calculation.

$\sum \left( xw + (180^2 - 360x)w\delta \right)$

Fix
Added the missing square to correctly calculate the weighted sum of squares.

$\sum \left( x^2w + (180^2 - 360x)w\delta \right)$