0031437: Modeling Data - Offset surface throws exception when trying to evaluate at singularity point

Add test case to prove the exception is avoided.

tests/bugs/moddata_3/bug31437

@@ -0,0 +1,21 @@
+puts "========"
+puts "0031437: Offset surface throws e x c e p t i o n when trying to evaluate at singularity point"
+puts "========"
+puts ""
+
+restore [locate_data_file bug31437.draw] s
+
+set dist 2.5
+offset result s -$dist
+
+svalue s 0 1 x1 y1 z1
+svalue result 0 1 x2 y2 z2
+
+set curDist2 [expr ([dval x1] - [dval x2])**2 + ([dval y1] - [dval y2])**2 + ([dval z1] - [dval z2])**2]
+if {abs($curDist2 - $dist**2) < 1.e-7} {
+  puts "OK: offset correct"
+} else {
+  puts "Error: offset is wrong"
+}
+
+checkview -display result -2d -path ${imagedir}/${test_image}.png