Cleanup of user_libs especially docs.

docs/source/references.rst

@@ -14,6 +14,7 @@ References
 .. [GIA1997] Giannopoulos, A. (1997). The investigation of Transmission-Line Matrix and Finite-Difference Time-Domain Methods for the Forward Problem of Ground Probing Radar, D.Phil thesis, Department of Electronics, University of York, UK. (http://etheses.whiterose.ac.uk/id/eprint/2443)
 .. [GIA2012] Giannopoulos, A. (2012). Unsplit implementation of higher order PMLs. Antennas and Propagation, IEEE Transactions on, 60(3), 1479-1485. (http://dx.doi.org/10.1109/tap.2011.2180344)
 .. [IRE2013] Ireland, D., & Abbosh, A. (2013). Modeling human head at microwave frequencies using optimized Debye models and FDTD method. Antennas and Propagation, IEEE Transactions on, 61(4), 2352-2355. (http://dx.doi.org/10.1109/tap.2013.2242037)
+.. [KEL2007] Kelley, D. F., Destan, T. J., & Luebbers, R. J. (2007). Debye function expansions of complex permittivity using a hybrid particle swarm-least squares optimization approach. IEEE Transactions on Antennas and propagation, 55(7), 1999-2005. (https://doi.org/10.1109/TAP.2007.900230)
 .. [KUN1993] Kunz, K. S., & Luebbers, R. J. (1993). The finite difference time domain method for electromagnetics. CRC press.
 .. [LI2013] Li, J., Guo, L. X., Jiao, Y. C., & Wang, R. (2013). Composite scattering of a plasma-coated target above dispersive sea surface by the ADE-FDTD method. Geoscience and Remote Sensing Letters, IEEE, 10(1), 4-8. (http://dx.doi.org/10.1109/lgrs.2012.2189751)
 .. [LUE1994] Luebbers, R., Steich, D., & Kunz, K. (1993). FDTD calculation of scattering from frequency-dependent materials. Antennas and Propagation, IEEE Transactions on, 41(9), 1249-1257. (http://dx.doi.org/10.1109/8.247751)