dense_lambda: Get a lambda process with more dense time step
In ksublee/mHawkes: Simulate and estimate marked Hawkes process

Since the intenisty process is exponencially decaying, to fully describe the intensity processs, we need more dense time line. This function gives a lambda process with more dense time line.

1	dense_lambda(arrival, lambda, beta, dt = NULL, ...)

`arrival`	a vector of arrival times.
`lambda`	a vector of lambda processs. It should be a right continuous version.
`beta`	a decaying parameter lambda.
`dt`	a small step size on time horizon.

MU1 <- 0.3; ALPHA1 <- 1.5; BETA1 <- 2
mhspec1 <- new("mhspec", MU=MU1, ALPHA=ALPHA1, BETA=BETA1)
# Simulate with mhsim funciton.
res1 <- mhsim(mhspec1,  n=100)
dl <- dense_lambda(res1$arrival, res1$lambda, BETA1)