Add facility to save plots in directory structure.

- Directories are named according to commit hash.

- Log scale has been removed from graphs due to a
  ValueError. Zero cannot be represented on a log
  scale.

tardis/tests/tests_slow/test_w7.py

@@ -5,6 +5,7 @@
 from numpy.testing import assert_allclose
 from astropy.tests.helper import assert_quantity_allclose
 
+import tardis
 from tardis.atomic import AtomData
 from tardis.simulation.base import Simulation
 from tardis.model import Radial1DModel
@@ -57,6 +58,11 @@ def setup(self, request, reference, data_path, atomic_data_fname,
         # Get the reference data through the fixture.
         self.reference = reference
 
+        # Path to directory where comparison plots for test run on current
+        # commit will be saved. Temporarily kept as `/tmp` directory.
+        # TODO: Can be created as a fixture later.
+        self.plot_savedir = os.path.join("/tmp", tardis.__githash__)
+
     def test_j_estimators(self):
         assert_allclose(
                 self.reference['j_estimators'],
@@ -145,14 +151,13 @@ def plot_spectrum(self, passed):
                         bbox={'facecolor': 'green', 'alpha': 0.5, 'pad': 10})
             ldl_ax.plot(self.reference['wavelength'], deviation, "g+")
         else:
-            ldl_ax.set_yscale("log")
             ldl_ax.text(0.8, 0.8, 'failed', transform=ldl_ax.transAxes,
                         bbox={'facecolor': 'red', 'alpha': 0.5, 'pad': 10})
             ldl_ax.plot(self.reference['wavelength'], deviation, "rx")
 
         # Figure is saved in `tmp` directory right now, till a suitable way of
         # saving them is decided.
-        plt.savefig(os.path.join("/tmp", "spectrum_plot.png"))
+        plt.savefig(os.path.join(self.plot_savedir, "spectrum.png"))
 
     def test_montecarlo_properties(self):
         assert_quantity_allclose(