Fix Student's T kernel formula sign error in tdist()

The tdist() function had a double-negative that caused strength to
increase with distance instead of decrease:

  return max / pow(base, -(nu + 1.0) / 2.0);  // wrong: dividing by negative exp

Fixed to:

  return max * pow(base, -(nu + 1.0) / 2.0);  // correct: multiply by negative exp

This bug caused the Student's T spatial kernel to produce physically
nonsensical results where interaction strength exceeded fmax at non-zero
distances.

Author: andrewkern

core/spatial_kernel.h

@@ -74,7 +74,7 @@ class SpatialKernel
 	// we don't use the GSL for this, because it does two gamma-function calculations that we don't need (they normalize away)
 	static inline double tdist(double x, double max, double nu, double tau) {
 		double x_over_tau = x / tau;
-		return max / pow(1.0 + x_over_tau * x_over_tau / nu, -(nu + 1.0) / 2.0);
+		return max * pow(1.0 + x_over_tau * x_over_tau / nu, -(nu + 1.0) / 2.0);
 	};
 
 public: