Initial value of pulse apparition time

[Ipuch]

https://github.com/Kev1CO/cocofest/blob/c43ce9463cf79ed9025240e666f50f70cac7e2d5/cocofest/optimization/fes_ocp.py#L572C13-L572C78

Subject of the enhancement

pulse apparition time initial guess is far from its final value.

Proposed enhancement

put the value between the bounds. (min+max/2)