Make the corner plots use 1, 2, and 3-sigma countours

Based on http://corner.readthedocs.io/en/latest/pages/sigmas.html This makes the countour levels 1, 2, and 3 sigma. Then the quantiles are 1 sigma.

Old behaviour:

New behaviour:

tupak/core/result.py

@@ -218,7 +218,8 @@ class Result(dict):
             bins=50, smooth=0.9, label_kwargs=dict(fontsize=16),
             title_kwargs=dict(fontsize=16), color='#0072C1',
             truth_color='tab:orange', show_titles=True,
-            quantiles=[0.025, 0.975], levels=(0.39, 0.8, 0.97),
+            quantiles=[0.16, 0.84],
+            levels=(1-np.exp(-0.5), 1-np.exp(-2), 1-np.exp(-9/2.)),
             plot_density=False, plot_datapoints=True, fill_contours=True,
             max_n_ticks=3)