don't fit close to 0K

cryogenic temperatures are normally not of interest, avoid considering
the strongly curved region below 80K in the fit gives probably better
extrapolation properties for T>300K

examples/config/phase/mechanical/elastic/Hooke_Sn-beta.yaml

@@ -4,29 +4,29 @@ references:
   - J.A. Rayne and B.S. Chandrasekhar,
     Physical Review 120(5):1658-1663, 1960,
     https://doi.org/10.1103/PhysRev.120.1658,
-    fit to Fig. 2 (T_min=40K, T_max=300K) and Tab. IV (C_13, T_min=77K, T_max=300K)
+    fit to Fig. 2 (T_min=100K, T_max=300K) and Tab. IV (C_13, T_min=77K, T_max=300K)
 
-C_11: 72.89e+9
-C_11,T: -4.479e+7
-C_11,T^2: -3.149e+4
+C_11: 72.90e+9
+C_11,T: -4.399e+7
+C_11,T^2: -2.645e+4
 
 C_12: 59.27e+9
-C_12,T: 1.110e+7
-C_12,T^2: 1.300e+4
+C_12,T: 1.058e+7
+C_12,T^2: 1.002e+4
 
 C_13: 35.80e+9
 C_13,T: -2.870e+6
 
-C_33: 88.77e+10
-C_33,T: -5.361e+7
-C_33,T^2: -2.770e+3
+C_33: 88.78e+9
+C_33,T: -5.250e+7
+C_33,T^2: 3.546e+3
 
 C_44: 22.26e+9
-C_44,T: -1.994e+7
-C_44,T^2: -9.519e+3
+C_44,T: -1.982e+7
+C_44,T^2: -8.711e+3
 
-C_66: 24.17e+9
-C_66,T: -1.919e+7
-C_66,T^2: -1.161e+4
+C_66: 24.18e+9
+C_66,T: -1.806e+7
+C_66,T^2: -4.112e+3
 
 T_ref: 293.15