Add STW105 reference model of Kustowski et al. (2008)

STW105 is the reference model for S362ANI and S362WMANI.
It takes Q from QL6 and matches PREM below the transition zone,
but is different above that.

The original is parameterised by raidal splines; this version
is taken from IRIS and is a LinearLayeredModel.

README.md

@@ -120,6 +120,7 @@ does support reading and writing of
   - `AK135`
   - `IASP91`
   - `PREM`
+  - `STW105`
 - Moon
   - `MOON_WEBER_2011`
 

docs/src/inbuilt_models.md

@@ -1,12 +1,17 @@
 # Inbuilt models
 
 Most users of SeisModels will probably want to simply calculate
-properties for one of the inbuilt models of the Earth and moon.
-They are:
+properties for one of the inbuilt models.  These are listed below:
 
+## Earth models
 ```@docs
 AK135
 IASP91
 PREM
+STW105
+```
+
+## Moon models
+```@docs
 MOON_WEBER_2011
-```
+```

docs/src/references.md

@@ -1,5 +1,8 @@
 # References
 
+- Durek, J.J., Ekström, G., 1996.
+  A radial model of anelasticity consistent with long-period surface-wave attenuation.
+  _Bull Seism Soc Am_ 86, 144–158.
 - Dziewoński, A., Anderson, D., 1981. Preliminary reference Earth model.
   _Phys. Earth Planet. Inter._ 25, 297–356. doi:[10.1016/0031-9201(81)90046-7](https://doi.org/10.1016/0031-9201(81)90046-7)
 - Kennett  B.L.N.  Engdahl  E.  1991. Traveltimes for global earthquake
@@ -8,6 +11,10 @@
 - Kennett, B.L.N., Engdahl, E., Buland, R., 1995. Constraints on seismic
   velocities in the Earth from travel-times.
   _Geophys J Int_ 122, 108–124. doi:[10.1111/j.1365-246X.1995.tb03540.x](https://doi.org/10.1111/j.1365-246X.1995.tb03540.x)
+- Kustowski, B., Ekström, G., Dziewoński, A., 2008.
+  Anisotropic shear-wave velocity structure of the Earth’s mantle: A global model.
+  _J Geophys Res-Sol Ea_ 113, B06306.
+  doi:[10.1029/2007JB005169](https://doi.org/10.1029/2007JB005169)
 - Weber, R.C.,, Lin, P.-Y.,, Garnero, E.J., Williams, Q., Lognonné, P., 2011.
   Seismic Detection of the Lunar Core. _Science_, 331, 309–312.doi:[10.1126/science.1199375](https://doi.org/10.1126/science.1199375)
 

src/Earth/Earth.jl

@@ -14,10 +14,11 @@ List of Earth model instances as `Symbol`s.
 
 If adding new inbuilt Earth models, append the name of the new model to this list
 """
-const ALL_MODELS = (:AK135, :IASP91, :PREM)
+const ALL_MODELS = (:AK135, :IASP91, :PREM, :STW105)
 
 include("ak135.jl")
-include("prem.jl")
 include("iasp91.jl")
+include("prem.jl")
+include("stw105.jl")
 
 end # module