Option for scecifying the evaluation strategy of the function that uses self.
SimpleState inteprets self as simply internal state of the function which is initialized as 0 at time = 0.
ZeroAtInit inteprets the return value of the function which uses self returns 0 at time = 0.
For example, | | {self + 1} will return 1 at time = 0 in SimpleState mode, but 0 in ZeroAtInit mode.
For most of the cases, SimpleState is the default and recommended mode.