add explicit instantiation of template method

apparently some versions of compilers need this

bfps/cpp/particles/particles_inner_computer.cpp

@@ -160,3 +160,9 @@ void particles_inner_computer<double, long long>::enforce_unit_orientation<6>(
     }
 }
 
+template
+void particles_inner_computer<double, long long>::compute_interaction<6, 6>(
+        const long long nb_particles,
+        const double pos_part[],
+        double rhs_part[]) const;
+