Gallium nitride

Add new medium class for GaN.

• Electron mobility model from F. Schwierz, Solid-State Electronics 49 (2005), 889.
• Low-field hole mobility from T. TMnatsakanov et al., Solid-State Electronics 47 (2003), 111
• High-field hole mobility parameterization from Synopsys Sentaurus.
• Impact ionization parameters from J. Baliga, Semicond. Sci. Technol. 28 (2013) 074011