Fix bug whereby coherent elastic scattering cross section was zero

src/thermal.cpp

@@ -202,7 +202,7 @@ ThermalScattering::calculate_xs(double E, double sqrtkT, int* i_temp,
   *inelastic = (1.0 - f) * xs[i_grid] + f * xs[i_grid + 1];
 
   // Check for elastic data
-  if (E < sab.threshold_elastic_) {
+  if (!sab.elastic_e_in_.empty()) {
     // Determine whether elastic scattering is given in the coherent or
     // incoherent approximation. For coherent, the cross section is
     // represented as P/E whereas for incoherent, it is simply P

tests/regression_tests/lattice_hex_coincident/results_true.dat

@@ -1,2 +1,2 @@
 k-combined:
-1.741370E+00 1.384609E-03
+1.744283E+00 1.734930E-03

tests/regression_tests/salphabeta/results_true.dat

@@ -1,2 +1,2 @@
 k-combined:
-8.403447E-01 2.461538E-02
+8.757212E-01 5.686265E-02

tests/regression_tests/triso/results_true.dat

@@ -1,2 +1,2 @@
 k-combined:
-1.707485E+00 9.795497E-02
+1.701412E+00 3.180877E-02

tests/regression_tests/void/results_true.dat

@@ -1,2 +1,2 @@
 k-combined:
-1.062505E+00 2.674375E-02
+9.612556E-01 1.990135E-02