Reorder corners coordinates for convenience

Previously one required that corners a and b were defined such that
a[i]<b[j]. It is no more the case.

Also check that `a[i]-b[j] != 0`. This avoids crashes which would be
difficult to track.

src/mesh/CartesianMeshBuilder.cpp

@@ -531,7 +531,23 @@ CartesianMeshBuilder::CartesianMeshBuilder(const TinyVector<Dimension>& a,
                                            const TinyVector<Dimension, uint64_t>& size)
 {
   if (parallel::rank() == 0) {
-    this->_buildCartesianMesh(a, b, size);
+    TinyVector lenght = b - a;
+    for (size_t i = 0; i < Dimension; ++i) {
+      if (lenght[i] == 0) {
+        throw NormalError("invalid box definition corners share a component");
+      }
+    }
+
+    TinyVector<Dimension> corner0 = a;
+    TinyVector<Dimension> corner1 = b;
+
+    for (size_t i = 0; i < Dimension; ++i) {
+      if (corner0[i] > corner1[i]) {
+        std::swap(corner0[i], corner1[i]);
+      }
+    }
+
+    this->_buildCartesianMesh(corner0, corner1, size);
   }
   this->_dispatch<Dimension>();
 }