Add a custom EulerAngles class

Include `EulerAngles` class in mrsimulator

```py
class EulerAngles:
    alpha: float
    beta: float
    gamma: float
```
Define a method `__add__` to add two Euler angle.

```py
e1 = EulerAngles(alpha=0.5, beta=3.14, gamma=1.14)
e2 = EulerAngles(alpha=1.2, beta=1.23, gamma=-2.0)
e3 = e1 + e2
```