Thomson cross section: Continuation of PR #688 (#863)

* Use sigma_T from astropy constants instead of hardcoding it, also fixing its unit.

* Use sigma_T constant in test_cmontecarlo instead of a hardcoded value.

* Change refdata branch

* Update .travis.yml

.travis.yml

@@ -78,8 +78,8 @@ before_install:
     - if [[ $TEST_MODE == 'spectrum' ]]; then git fetch origin; fi
     - if [[ $TEST_MODE == 'spectrum' ]]; then git checkout origin/master; fi
     # Use the following to get the ref-data from a specific pull request; 
-    # - if [[ $TEST_MODE == 'spectrum' ]]; then git fetch origin pull/10/head:formalintegral-ref; fi
-    # - if [[ $TEST_MODE == 'spectrum' ]]; then git checkout formalintegral-ref; fi
+    # - if [[ $TEST_MODE == 'spectrum' ]]; then git fetch origin pull/11/head:thomson-ref; fi
+    # - if [[ $TEST_MODE == 'spectrum' ]]; then git checkout thomson-ref; fi
     - if [[ $TEST_MODE == 'spectrum' ]]; then git lfs pull --include="atom_data/kurucz_cd23_chianti_H_He.h5" origin; fi
     - if [[ $TEST_MODE == 'spectrum' ]]; then git lfs pull --include="atom_data/chianti_He.h5" origin; fi
     - if [[ $TEST_MODE == 'spectrum' ]]; then git lfs pull --include="plasma_reference/" origin; fi

tardis/montecarlo/base.py

@@ -391,10 +391,10 @@ def from_config(cls, config):
         """
         if config.plasma.disable_electron_scattering:
             logger.warn('Disabling electron scattering - this is not physical')
-            sigma_thomson = 1e-200 / (u.cm ** 2)
+            sigma_thomson = 1e-200 * (u.cm ** 2)
         else:
             logger.debug("Electron scattering switched on")
-            sigma_thomson = 6.652486e-25 / (u.cm ** 2)
+            sigma_thomson = const.sigma_T.cgs
 
         spectrum_frequency = quantity_linspace(
             config.spectrum.stop.to('Hz', u.spectral()),

tardis/montecarlo/tests/test_cmontecarlo.py

@@ -47,6 +47,7 @@
 import pytest
 import numpy as np
 import pandas as pd
+from astropy import constants as const
 from ctypes import (
         CDLL,
         byref,