Simulate a Hawkes process

The submission considers first-order exponential-decay Hawkes processes with constant unconditional intensities, and presents
a) a calculation of point-time intensities for constituent series of a multivariate process;

b) a plotting function visualizing intensity dynamics and event occurrences of a multivariate process;

c) a *univariate* process simulator, based on Algorithm 1 of Bravaccino (2004, p. 80). A multivariate simulator is desired, and collaboration is enthusiastically welcomed.