Add force calculation kernel and fix incorrect ctest configuration for Cuda tests

kernels/forces.cuh

@@ -0,0 +1,19 @@
+#ifndef FORCES_CUH
+#define FORCES_CUH
+
+#include "pair_potentials.cuh"
+#include "precision.hpp"
+namespace CAC {
+/**
+ * Calculate forces and energies using CUDA for acceleration
+ * This code currently only accepts a single PairPotential object and does an
+ * n^2 force calculation. Future improvements will:
+ *   - Allow for neighbor listing
+ *   - Allow for overlaid force calculations
+ */
+__global__ void calc_forces_and_energies(real *xs, real *forces, real *energies,
+                                         int n_particles, real *box_bd,
+                                         PairPotential &potential);
+} // namespace CAC
+
+#endif