Use math.radians and math.degrees consistently

[Jie-F]

In the code, there are two places that use the manual thetapi/180 or theta180/pi calculations. This is slower than the math.radians or math.degrees which only does one floating point multiplication in C, compared to one float multiplication and float division in Python, which is over twice as slow. Additionally, the two conversion methods produce slightly different results due to floating point imprecision.