Fixed missing import & added plt.show() calls.

Existing hawkes.py refers to a 'plt' variable, but this must
first be imported to be used. Also, for the plots to display
two separate plt.show() calls are added

Note: the first plots must be closed before the last is shown,
otherwise the final QQ plot is drawn on top of the previous plot).

hawkes.py

@@ -44,6 +44,7 @@ def rate_series(t, x):
 
 ## plot settings
 import matplotlib as mpl
+import matplotlib.pyplot as plt
 mpl.rc('font', **{'sans-serif':'Verdana','family':'sans-serif','size':8})
 mpl.rcParams['xtick.direction'] = 'out'
 mpl.rcParams['ytick.direction'] = 'out'
@@ -84,7 +85,7 @@ def rate_series(t, x):
 ax.set_title('Fitted vs Empirical Intensities')
 
 plt.draw()
-
+plt.show()
 
 ## QQ plot of residuals
 import scipy.stats as stats
@@ -94,4 +95,4 @@ def rate_series(t, x):
 ax.set_title('Residual Interevent Times')
 
 plt.draw()
-
+plt.show()