integral_intensity: Computes term two of the log likelihood
In mrc-ide/epihawkes: Hawkes Processes model for outbreak analysis

Term two of the log likelhood function is:

\int_{0}^{T} μ(τ) dτ - ∑_{i=1}^{n} \int_{t_{i}}^{T} g(τ - t_{i} dτ)

integral_intensity(
  events,
  int_kernel,
  parameters,
  mu_fn = mu_none,
  mu_diff_fn = mu_diff_none,
  mu_int_fn = mu_int_none,
  print_level = 1
)

`events`	Vector of event times.
`int_kernel`	Integral of kernel function for Hawkes Process.
`parameters`	Parameters of the Hawkes kernel.
`mu_fn`	Function that returns exogenous part of Hawkes Process.
`mu_diff_fn`	Function that returns differential of exogenous part.
`mu_int_fn`	Function that returns integral of exogenous part.
`print_level`	Level at which logger will print