put arb_cell_description in arb namespace (#445)

Added cell description to the Arbor namespace.

src/lif_cell_description.hpp

 #pragma once
 
+namespace arb {
+
 // Model parameteres of leaky integrate and fire neuron model.
 struct lif_cell_description {
     // Neuronal parameters.
@@ -11,3 +13,5 @@ struct lif_cell_description {
     double V_reset = E_L; // Reset potential [mV].
     double t_ref = 2;     // Refractory period [ms].
 };
+
+} // namespace arb

src/lif_cell_group.hpp

@@ -10,6 +10,7 @@
 #include <vector>
 
 namespace arb {
+
 class lif_cell_group: public cell_group {
 public:
     using value_type = double;
@@ -50,4 +51,5 @@ private:
     // Time when the cell was last updated.
     std::vector<time_type> last_time_updated_;
 };
+
 } // namespace arb