thermal expansion coefficients for SiC

this material was the initial motivation for adding fourth order
polynomials, but I could not find a reliable source for the graphs send
to damask@mpie.de. It seems that the CTE levels off for even higher
temperature which calls for a higher order polynomial

PRIVATE

@@ -1 +1 @@
-Subproject commit b258fe86c2fb4bc74e5649a100328a38bcb3c997
+Subproject commit 547bfe56d67cba358a7bb9582f2f7c0e344befd8

examples/config/phase/mechanical/eigen/thermalexpansion_SiC-alpha.yaml

@@ -0,0 +1,16 @@
+type: thermalexpansion
+
+references:
+  - Z. Li and R.C. Bradt,
+    Journal of the American Ceramic Society 69(12):863-866, 1986,
+    https://doi.org/10.1111/j.1151-2916.1986.tb07385.x
+
+Alpha_11: 3.27e-6
+Alpha_11,T: 3.25e-9
+Alpha_11,T^2: -1.36e-12
+
+Alpha_33: 3.18e-6
+Alpha_33,T: 2.48e-9
+Alpha_33,T^2: -8.51e-13
+
+T_ref: 273.15

examples/config/phase/mechanical/eigen/thermalexpansion_SiC-beta.yaml

@@ -0,0 +1,12 @@
+type: thermalexpansion
+
+references:
+  - Z. Li and R.C. Bradt,
+    Journal of Materials Science 21:4366-4368, 1986,
+    https://doi.org/10.1007/BF01106557
+
+Alpha_11: 3.19e-6
+Alpha_11,T: 3.60e-9
+Alpha_11,T^2: -1.68e-12
+
+T_ref: 273.15