Better error/help message for gmsh2tetgen (#536)

* DOC: Added a more descriptive error message when no inputs are provided to gmsh2tetgen

* DOC: Added an additional string on the length scale parameter in the gmsh2tetgen script

Author: zhang-alvin

scripts/gmsh2tetgen.f

@@ -57,7 +57,15 @@ program gmsh2tetgen
 	    call getarg(5,perm_z_string)           
 	    read (perm_z_string,*) perm(3)  
 	  else
-	    write(*,*) 'Wrong input'
+	    write(*,*) 'Input should be: (.mesh) (length scale, optional) '// &
+        '(x permutation, optional) (y permutation, optional) '// &
+        '(z permutation, optional)'
+      write(*,*) 'The length scale scales the vertex coordinates '// &
+        'in each dimension in the resulting mesh'
+      write(*,*) 'If using the optional permutation input, all three '//&
+        'permutations must be specified. The permutations dictate '//&
+        'how vertices are written out in the resulting mesh. Default '//&
+        'is [1,2,3]'
 	    stop
 	  endif