taken care of the case thatan x-array with 2 entries only needs to be splined...

taken care of the case thatan x-array with 2 entries only needs to be splined by using the natural_spline and not the estimated derivative spline method by default in this particular case

tools/arrays.c

@@ -589,6 +589,8 @@ int array_spline_table_lines(
     return _FAILURE_;
   }
 
+  if (x_size==2) spline_mode = _SPLINE_NATURAL_; // in the case of only 2 x-values, only the natural spline method is appropriate, for _SPLINE_EST_DERIV_ at least 3 x-values are needed.
+
 
   index_x=0;
 
@@ -749,6 +751,8 @@ int array_logspline_table_lines(
     return _FAILURE_;
   }
 
+  if (x_size==2) spline_mode = _SPLINE_NATURAL_; // in the case of only 2 x-values, only the natural spline method is appropriate, for _SPLINE_EST_DERIV_ at least 3 x-values are needed.
+
 
   index_x=0;
 
@@ -910,6 +914,8 @@ int array_spline_table_columns(
     return _FAILURE_;
   }
 
+  if (x_size==2) spline_mode = _SPLINE_NATURAL_; // in the case of only 2 x-values, only the natural spline method is appropriate, for _SPLINE_EST_DERIV_ at least 3 x-values are needed.
+
   index_x=0;
 
   if (spline_mode == _SPLINE_NATURAL_) {
@@ -1079,6 +1085,8 @@ int array_spline_table_columns2(
     return _FAILURE_;
   }
 
+  if (x_size==2) spline_mode = _SPLINE_NATURAL_; // in the case of only 2 x-values, only the natural spline method is appropriate, for _SPLINE_EST_DERIV_ at least 3 x-values are needed.
+
 #pragma omp parallel                                                \
   shared(x,x_size,y_array,y_size,ddy_array,spline_mode,p,qn,un,u)   \
   private(index_y,index_x,sig,dy_first,dy_last)
@@ -1201,6 +1209,8 @@ int array_spline_table_one_column(
     return _FAILURE_;
   }
 
+  if (x_size==2) spline_mode = _SPLINE_NATURAL_; // in the case of only 2 x-values, only the natural spline method is appropriate, for _SPLINE_EST_DERIV_ at least 3 x-values are needed.
+
   /************************************************/
 
   index_x=0;
@@ -1334,6 +1344,8 @@ int array_logspline_table_one_column(
     return _FAILURE_;
   }
 
+  if (x_size==2) spline_mode = _SPLINE_NATURAL_; // in the case of only 2 x-values, only the natural spline method is appropriate, for _SPLINE_EST_DERIV_ at least 3 x-values are needed.
+
   /************************************************/
 
   index_x=0;