Updated helium nlte page with in-text citation, and added that paper …

…to the references page

docs/physics/setup/plasma/helium_nlte.rst

@@ -3,7 +3,7 @@ Helium NLTE
 
 The `helium_treatment` setting in the TARDIS config. file will accept one of three options:
  * `none`: The default setting. Populate helium in the same way as the other elements.
- * `recomb-nlte`: Treats helium in NLTE using the analytical approximation outlined in an upcoming paper. 
+ * `recomb-nlte`: Treats helium in NLTE using the analytical approximation outlined in :cite:`Boyle2017`. 
  * `numerical-nlte`: To be implemented. Will allow the use of a separate module (not distributed with TARDIS) to perform helium NLTE calculations numerically.
 
 Recombination He NLTE

docs/tardis.bib

@@ -331,3 +331,20 @@ @ARTICLE{Ore1949
        adsurl = {https://ui.adsabs.harvard.edu/abs/1949PhRv...75.1696O},
       adsnote = {Provided by the SAO/NASA Astrophysics Data System}
 }
+
+@ARTICLE{Boyle2017,
+       author = {{Boyle}, Aoife and {Sim}, Stuart A. and {Hachinger}, Stephan and {Kerzendorf}, Wolfgang},
+        title = "{Helium in double-detonation models of type Ia supernovae}",
+      journal = {\aap},
+     keywords = {supernovae: general, white dwarfs, radiative transfer, Astrophysics - High Energy Astrophysical Phenomena, Astrophysics - Solar and Stellar Astrophysics},
+         year = 2017,
+        month = mar,
+       volume = {599},
+          eid = {A46},
+          doi = {10.1051/0004-6361/201629712},
+archivePrefix = {arXiv},
+       eprint = {1611.05938},
+ primaryClass = {astro-ph.HE},
+       adsurl = {https://ui.adsabs.harvard.edu/abs/2017A&A...599A..46B},
+      adsnote = {Provided by the SAO/NASA Astrophysics Data System}
+}