gen_hawkes_process: Generate Hawkes Process
In wush978/RPPGen: Generator of Point Process

The intensity of generated process is

λ(t) + ∑_{t_i < t} phi(t - t_i)

.

1 2	gen_hawkes_process(lambda, T_0, lambda_u = NULL, phi = dexp, phi_u = NULL)

`lambda`	a positive function.
`T_0`	positive numeric value. The generated events in `[0, T_0]`
`lambda_u,`	numeric value. The upper bound of `lambda(t)` in `[0, T_0]`. If `lambda_u` is `NULL`, then `optimize` will be used to find the upper bound.
`phi`	a positive function, which is the self-excited intensity effect.