models: Add doi links to references

Adds the `doi` links to the reference to make the publications easier to
find and access.

src/eppaurora/models/__init__.py

@@ -12,7 +12,7 @@
 providing the proxy driven ionization rate model described in [1]_
 and the empirical electron energy and flux model described in [2]_.
 
-.. [1] Bender et al., in prep., 2023
+.. [1] Bender et al., in prep., 2023, preprint: https://doi.org/10.48550/arXiv.2312.11130
 .. [2] Zhang and Paxton, JASTP, 70, 1231--1242, 2008, https://doi.org/10.1016/j.jastp.2008.03.008
 """
 

src/eppaurora/models/ssusiq2023.py

@@ -11,7 +11,7 @@
 Implements the empirical model for auroral ionization,
 derived from SSUSI UV observations [1]_.
 
-.. [1] Bender et al., in prep., 2023
+.. [1] Bender et al., in prep., 2023, preprint: https://doi.org/10.48550/arXiv.2312.11130
 """
 from logging import warning as warn
 from os import path

src/eppaurora/models/zhangpaxton2008.py

@@ -11,7 +11,7 @@
 Implements the empirical Kp-driven model for auroral electrons,
 providing mean energy and energy flux as described in [1]_.
 
-.. [1] Zhang and Paxton, JASTP, 70, 1231--1242, 2008
+.. [1] Zhang and Paxton, JASTP, 70, 1231--1242, 2008, https://doi.org/10.1016/j.jastp.2008.03.008
 """
 from os import path
 from pkg_resources import resource_filename