Add linestyle option for plot_multiple

Sometimes one may wish to use linestyles as well as colors to differentiate results plotted together with plot_multiple. This merge request adds that functionality.

In the process, I found that hist_kwargs and label_kwargs would not update properly in plot_corner, overwriting defaults instead of updating them. This MR now fixes that issue as well.

bilby/core/result.py

@@ -1189,7 +1189,6 @@ class Result(object):
             defaults_kwargs['truth_color'] = 'tab:blue'
             defaults_kwargs['color'] = '#FF8C00'
 
-
         label_kwargs_defaults = dict(fontsize=16)
         hist_kwargs_defaults = dict(density=True)
 
@@ -2042,7 +2041,7 @@ def plot_multiple(results, filename=None, labels=None, colours=None,
         hist_kwargs['color'] = c
         hist_kwargs["linestyle"] = linestyle
         kwargs["hist_kwargs"] = hist_kwargs
-        fig = result.plot_corner(fig=fig, save=False, color=c, contour_kwargs={"linestyle":linestyle}, **kwargs)
+        fig = result.plot_corner(fig=fig, save=False, color=c, contour_kwargs={"linestyle": linestyle}, **kwargs)
         default_filename += '_{}'.format(result.label)
         lines.append(mpllines.Line2D([0], [0], color=c, linestyle=linestyle))
         default_labels.append(result.label)