Fix #949: compiler errors of egs_mesh on mac

Fix the used of [] instead of .at() for arrays in egs_mesh, and include
the array library explicitly. This avoids compiler errors that occurred
on mac.

HEN_HOUSE/egs++/geometry/egs_mesh/egs_mesh.h

@@ -59,6 +59,7 @@
 #include <memory>
 #include <sstream>
 #include <vector>
+#include <array>
 
 #include "egs_base_geometry.h"
 #include "egs_vector.h"
@@ -306,10 +307,10 @@ class EGS_MESH_EXPORT EGS_Mesh : public EGS_BaseGeometry {
  << " \t2: " << n.C.x << " " << n.C.y << " " << n.C.z << "\n"
  << " \t3: " << n.D.x << " " << n.C.y << " " << n.C.z << "\n"
  << " \tNeighbour elements:\n"
- << " \t\tOn face 0: " << neighbours_[i][0] << "\n"
- << " \t\tOn face 1: " << neighbours_[i][1] << "\n"
- << " \t\tOn face 2: " << neighbours_[i][2] << "\n"
- << " \t\tOn face 3: " << neighbours_[i][3] << "\n"
+ << " \t\tOn face 0: " << neighbours_.at(i).at(0) << "\n"
+ << " \t\tOn face 1: " << neighbours_.at(i).at(1) << "\n"
+ << " \t\tOn face 2: " << neighbours_.at(i).at(2) << "\n"
+ << " \t\tOn face 3: " << neighbours_.at(i).at(3) << "\n"
  << std::boolalpha
  << " \tBoundary element: " << is_boundary(i) << "\n"
  << " \tMedia index: "<< medium_indices_[i] << "\n";
@@ -363,10 +364,10 @@ class EGS_MESH_EXPORT EGS_Mesh : public EGS_BaseGeometry {
  Nodes element_nodes(int element) const {
  const auto &node_indices = elt_node_indices_.at(element);
  return Nodes {
- nodes_.at(node_indices[0]),
- nodes_.at(node_indices[1]),
- nodes_.at(node_indices[2]),
- nodes_.at(node_indices[3])
+ nodes_.at(node_indices.at(0)),
+ nodes_.at(node_indices.at(1)),
+ nodes_.at(node_indices.at(2)),
+ nodes_.at(node_indices.at(3))
  };
  }